On October 14th/15th 1969 the "Zodiac Killer" made a phone call to Santa Rosa claiming he was going to place bombs on school buses, thereby proving this was from the real killer of five people. This call came before the contents of the October 13th 1969 letter threatening schoolchildren was released into the public domain. In fact, this was the first mention of bombs, nearly a month before the Zodiac Killer's "Bus Bomb" letter was mailed to the San Francisco Chronicle on November 9th 1969. However, this probably wasn't the only time the Zodiac Killer threatened to blow up school buses prior to the arrival of the "Bus Bomb" letter. The second set of phone calls threatening school buses likely began in the latter days of October or early November, and continued for several weeks, before culminating on November 24th 1969.

These threats were made to the Lake Tahoe region and Incline Village, the location featured in the Zodiac Killer's "Pines" card on March 22nd 1971. It must be noted that these threats began seven months before Donna Lass relocated from the Letterman General Hospital in the Presidio to South Lake Tahoe. The newspaper article below is from the Sacramento Bee on November 24th 1969, describing a series of malicious calls "received during the last few weeks". If we take a "few weeks" to mean 3 or 4 weeks, it would suggest that these calls began on November 3rd 1969 at the latest, and continued throughout the duration the Zodiac Killer was composing his "Bus Bomb" letter. In other words, if you believe the Lake Tahoe school bus threats were made by a hoaxer (and don't believe the Zodiac Killer made the Santa Rosa phone calls in October), you are left with an individual mimicking the Zodiac Killer's bomb threats before the Zodiac Killer even made any bomb threats on school buses himself.    

We likely now have two instances of bus bomb threats being made upon schoolchildren from somebody claiming to be the "Zodiac Killer", prior to the first mention of bombs in the verified "Bus Bomb" letter on November 9th 1969. But don't expect this to sway the majority of the Zodiac community into deviating from the well worn narrative they have become accustomed to over the years. They are fixated and unflinching in the belief we have only four Zodiac ciphers, two Zodiac phone calls, and about twenty Zodiac letters. No amount of evidence will ever change this.

The "Bus Bomb" letter and Lake Tahoe phone calls coincided with a two week threat towards school teacher Daniel Williams in Martinez (from October 22nd to November 5th 1969), in which he endured malicious phone calls from the "Zodiac Killer" and an attempted poisoning by arsenic. The phone caller stated he "had gone to a Martinez school in search of victims but left when he found police there", but they wouldn't catch him because, as he stated “I’m too smart for them". A few days later, the Zodiac Killer mailed the "Bus Bomb" letter bragging that "The police shall never catch me, because I have been too clever for them". He also supplied crosshairs with five X's on its circumference, which when tilted close to magnetic north, landed over every crime he was claiming thus far, apart from the X on the 9 o' clock position. When a line was drawn through this position, it traveled directly over the Salesian High School where Daniel Williams worked as a teacher. We now have three occurrences in early November 1969 - all related to the targetting of school buses and/or schoolchildren. It's amazing how many of these hoaxers knew what the Zodiac Killer was going to do before he did. Quite amazing.    

In the previous article I covered William Frederick Friedman and the "CSP 888" cipher machine, a highly secure, electromechanical rotor-based cipher machine used for top-level communications during World War II. Friedman was instrumental in leading the Army's cryptologic services during the 1920s and 1930s, which set the stage for the development of secure machines like the CSP 888. There is much reason to connect Edgar Allan Poe to the design of the Zodiac Killer's Z340 and Z13 through his cryptographic works, such as Poe's July 1841 essay "A Few Words on Secret Writing", which William Friedman mentioned in his 1937 bulletin entitled “Edgar Allen Poe, Cryptographer”.

​In Poe's essay, the alphabet was split A through M and N through Z, which was seemingly adopted by the Zodiac Killer on April 20th 1970, beginning and ending his ciphertext characters with the letters A and M, in an array of characters totalling thirteen (half an alphabet). To then find the three circled 8's present in the cipher and possibly mimicking the cipher machine "CSP 888", was extremely interesting (whether deliberate or accidental). Especially when you consider that William Friedman and Edgar Allan Poe are bound together by the words in “Edgar Allen Poe, Cryptographer” (1937), written by Friedman, whose career in the field of cryptology was originally inspired by Edgar Allan Poe. Was the Zodiac Killer influenced by the cryptographic work of William Friedman, who drew upon his knowledge of this individual when he was challenged by Donald C. B. Marsh of the American Cryptogram Association inviting the Zodiac Killer to mail a cipher which truly included his name?  A challenge published in the San Francisco Examiner on October 22nd 1969, just 17 days before the Zodiac Killer mailed his infamous 340 cipher on November 8th 1969, which may also have been influenced by the works of William Friedman.

The 340 cipher was finally cracked in December 2020 by David Oranchak, Sam Blake and Jarl Van Eycke, who discovered that the cipher was a combination of substitution and transposition (using frequency analysis). The Riverbank Publications is a series of pamphlets written by the people who worked for millionaire George Fabyan in the multi-discipline research facility he built in the early 20th century near Chicago. They were published by Fabyan, often without author credit. The publications on cryptanalysis, mostly written by William Friedman, with contributions from Elizebeth Smith Friedman and others, are considered seminal in the field. In particular, Publication 22 introduced the Index of Coincidence, a powerful statistical tool for cryptanalysis. 

​William Friedman's work on transposition ciphers at Riverbank Laboratories (circa 1916–1920) established foundational, mathematical techniques for solving complex reordering ciphers. Notably, Riverbank Publication No. 19, Formulae for the Solution of Geometrical Transposition Ciphers, provided analytical methods to solve transposition, which he compared to reassembling jigsaw puzzle pieces. It's quite ironical that the title of Friedman's work was called Riverbank Publication No. 19, Formulae for the Solution of Geometrical Transposition Ciphers, when a period 19 shift was required to break the Zodiac Killer's 340 transposition cipher. This was a fraction of the work issued by William Friedman. But was he the inspiration for the Zodiac Killer, who employed this type of cipher on November 8th 1969? 

​Unfortunately, there is one final intriguing aspect to this story - and sadly it's very final. Just six days before the 340 cipher was mailed on November 8th 1969, William Frederick Friedman died of a heart attack at the age of 78. He passed away on November 2nd 1969, about a week before the Zodiac Killer's transposition cipher arrived at the San Francisco Chronicle. He was buried with full military honors at Arlington Cemetery on November 5th 1969. 

THE 888 CIPHER MACHINE USED IN WORLD WAR II

The Zodiac Killer was riding high in October 1970, currently sitting on four unbroken ciphers he had mailed on November 8th 1969, December 7th 1969, April 20th 1970 and June 26th 1970. On October 6th 1970 the Zodiac Killer would mail the 13-Hole "Punch Card" with thirteen holes punched through the fabric of the card, while simultaneously declaring himself "crackproof". His use of the words "crack" or "cracked" were usually chosen when referring to his ciphers, so one could conclude that the thirteen punch holes in the card, having been mailed 5 1/2 months after his 13-Symbol cipher on April 20th 1970, which contained the words "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you? My name is....", could be related.

​Creating thirteen punch holes in the card (previously used in coding machines), in combination with the word "crackproof", may suggest a link between the two communications mailed on April 20th 1970 and October 6th 1970. Especially when you consider no cipher was included in the card mailed in October. It has previously been shown that the 13 punch holes were positioned in a 10:3 configuration, with the 3 punch holes placed directly below where they sat in the 13-Symbol cipher on April 20th 1970 (see below). If these two communications were linked together, we need to examine the Zodiac Killer's decision to use punch holes, his choice of the word "crackproof", and his selection of the three circled 8's in his 13-Symbol cipher, to see if we can connect all three to one encipherment technique or coding machine.  

The Zodiac Killer was "about 40" years of age in 1969 according to the three sets of eyewitnesses at Presidio Heights, so he would have been around 16 (or possibly slightly older) at the time World War II ended. If he had an interest in cryptography, he would almost certainly have known about Alan Turing, a British mathematician who led the crucial World War II effort at Bletchley Park, England to crack the German Enigma machine, an electromechanical rotor device used for secure military communications. This cipher machine is arguably the most famous cipher machine in the world, but it was far from unbreakable. 

It was clear to US cryptographers well before World War II that the single-stepping mechanical motion of rotor machines (e.g. the Hebern machine) could be exploited by attackers. In the case of the famous Enigma machine, these attacks were supposed to be upset by moving the rotors to random locations at the start of each new message. This, however, proved not to be secure enough, and German Enigma messages were frequently broken by cryptanalysis during World War II. William Friedman, director of the US Army's Signals Intelligence Service, devised a system to correct for this attack by truly randomizing the motion of the rotors. His modification consisted of a paper tape reader from a teletype machine attached to a small device with metal "feelers" positioned to pass electricity through the holes. When a letter was pressed on the keyboard the signal would be sent through the rotors as it was in the Enigma, producing an encrypted version. In addition, the current would also flow through the paper tape attachment, and any holes in the tape at its current location would cause the corresponding rotor to turn, and then advance the paper tape one position. In other words, the punch holes were a key ingredient to the encipherment process. link.

SIGABA (also known as the ECM Mark II) was primarily developed by a team of American cryptographers in the mid-1930s, with key contributions from William Friedman. The device was an electromechanical rotor-based cipher machine developed in the late 1930s in the United States as a joint effort of the US Army and US Navy  At the time it was considered a superior cipher machine, intended to keep high-level communications absolutely secure. It was used throughout WWII and was so reliable that it was used well into the 1950s, after which it was replaced by newer machines like AFSAM-7 (KL-7). As far as we know, SIGABA was never broken by Axis powers. In other words it was the primary encryption technique used in World War II by America - and to the best of our knowledge - remained "crackproof" throughout its service.

This gives us the "punch holes" in paper and the "crackproof" elements, but where do we find the "888" present in the 13-Symbol cipher, that was possibly highlighted in the 13-Hole "Punch Card"? The U.S. Army called the cipher machine SIGABA or Converter M-134. The U.S. Navy called the machine the CSP-888 (Cryptographic Security Publication) and CSP-889. The machine has three banks of 5 rotors each. The main bank (at the rear) holds 5 rotors with 26 contacts each. These are the main cipher rotors. They work in a similar way as the rotors of the con­temporary German Enigma machine.

The 13-Symbol cipher on April 20th 1970, many believe was a response to the challenge by Donald C. B. Marsh, who told the San Francisco Examiner on October 22nd 1969: "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in the cipher to established cryptogram experts. He knows, to quote Edgar Allan Poe, that any cipher created by man can be solved by man. Zodiac has not told the truth in his cipher messages to the Examiner, the Chronicle and the Vallejo Times-Herald. Zodiac has not done this, because to tell the complete truth in relation to his name, in cipher code, would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code, however complicated, which will truly and honestly include his name". 

By invoking the name of Edgar Allan Poe, it was probably hoped that the Zodiac Killer would use one of the cryptographic techniques described (or used) by Poe in his famous works, such as "A Few Words on Secret Writing", which contained all the ingredients used in the decryption of the 340 cipher and the design of the 13-character code, including the scytale method of decryption, the splitting of the alphabet, A throgh M and N through Z, and the cipher wheel, which uncovers a potential message in the Z13 code. The SIGABA cipher machine has room for three rows (or banks), of five cipher wheels each.

​In total, we have 13 punch holes and 13 ciphertext characters, a famous "crackproof" cipher machine and the same declaration by the Zodiac Killer on October 6th 1970, the 10:3 configuration on the "Punch Card" possibly highlighting the position of "888" as the key in the April 20th 1970 letter, the CSP-888 cipher machine used by the U.S. Navy in World War II, the cipher wheel explained by Edgar Allan Poe in July 1841 in A Few Words on Secret Writing, prompted by cryptographer Donald C. B. Marsh in a newspaper article on October 22nd 1969, and the signature of "ME⊕" revealed in the Z13 by using three right shifts of 8, which was also present in the Exorcist letter on January 29th 1974. The other possibility, is we have nothing. 

`` Edgar Allan Poe (1809–1849) popularized cryptography in the 1840s through stories like "The Gold-Bug," prompting a young William Friedman (1891–1969) to pursue the field. While Friedman became a legendary U.S. Army codebreaker who valued Poe’s role as a catalyst, he famously critiqued Poe’s actual cryptanalytic skills as amateurish in his 1936 analysis, "Edgar Allan Poe, Cryptographer". Could we therefore have a crossover between the cipher machine CSP-888 and the essay of Edgar Allan Poe, A Few Words on Secret Writing, manifesting itself on April 20th 1970? A Few Words on Secret Writing is mentioned by William Friedman on page 146 of his analysis (see below).  

​​William F. Friedman on Edgar Allan Poe
THE FRIEDMAN LECTURES ON CRYPTOLOGY

FOLLOW UP ARTICLE: A FEW DAYS AFTER DEATH 

It was common for the Zodiac Killer to compose letters and cards inspired by newspaper articles he had recently read, so it wouldn't have been out of the ordinary for the murderer of five to have been a regular consumer of the San Francisco Chronicle, including the games and puzzles section. On November 8th 1969 the Zodiac Killer mailed his infamous 340 cipher, unsolved for 51 years, until it was cracked by David Oranchak, Sam Blake and Jarl Van Eycke in 2020 using a knight's move (period 19) decryption technique. Could the Zodiac Killer have had an avid interest in chess, followed the popular column by George Koltanowski in the San Francisco Chronicle and incorporated a shift move into his encoding of the 340 cipher, that required a knight's move to unlock its secrets half a century later? Did the individual outlined below have an influence on the Zodiac Killer? The answer may lie in a newspaper article in the San Francisco Examiner on July 16th 1978, announcing the upcoming World Chess Championship beginning on July 18th 1978 with a match between ​Anatoly Karpov and Viktor Korchnoi, covered by guest speaker George Koltanowski at the Paul Masson Vineyard in Saratoga, Santa Clara County. A county where only 3 months earlier, the Zodiac Killer may have mailed the April 24th 1978 "I am back with you" letter from.   

George Koltanowski, the legendary grandmaster of chess who wrote more than 19,000 chess columns for The San Francisco Chronicle with the same ease with which he dispatched countless opponents in a career that spanned 10 decades, died Saturday (February 5th 2000) in a San Francisco hospital after a brief illness. He was 96. His column, which appeared in The Chronicle every day without interruption for 52 years, was the longest-running daily chess column in history.

He was also the world champion of a form of the game known as blindfold chess, in which the player commits the game to memory and does not look at the board or touch the pieces used by opponents, who play in the normal fashion. Mr. Koltanowski's 1937 feat of playing 34 opponents simultaneously while blindfolded without losing a game has never been equaled. Koltanowski was the former president of the U.S. Chess Federation, which bestowed upon him the title of "Dean of American Chess." He served during the years after the Bobby Fischer boom of 1972, when interest in chess soared to record highs after the mercurial American grandmaster won the world title. Koltanowski seized the momentum of those heady days to install chess clubs in countless schools, community centers and even at San Quentin Prison. San Francisco Chronicle.

Among his countless chess feats, Mr. Koltanowski was well-known for performing the Knight's Tour, where random bits of information such as names and phone numbers would be supplied by audience members and written in the 64 squares of a giant chessboard. In seconds, Mr. Koltanowski would commit the entire hodgepodge to memory. Then, while blindfolded, he would call out the intricate path required for a chess knight to make its series of L-shaped hops around the board -- by recalling the scraps of information in order.  ​If the Zodiac Killer had a liking or fascination with chess and puzzles, he would very likely have known about the knight's tour challenge, a mathematical problem where you have to move a knight around a chessboard (8 X 8 grid) and visit all 64 squares without landing on a square more than once. The Knight's Tour is very well-known, recognized as a classic, over-thousand-year-old mathematical puzzle and graph theory problem, first appearing in 9th-century Sanskrit texts and later analyzed by Leonhard Euler, becoming famous for its complexity and links to magic squares, popularizing it through chess history and computer science as an illustration of algorithms. ​

To commemorate the opening of the World Chess Championship in the Philippines on July 18th 1978, George Koltanowski appeared at a chess tournament in the Paul Masson Vineyard in Saratoga, Santa Clara County, which offered $15,000 in prize money and was open to 700 players. A chess tournament was held there annually in the late 1970s and early 1980s. The San Francisco Examiner newspaper reported on July 16th 1978 that it had not yet been decided whether George Koltanowski would be performing his mind-blowing Knight's Tour chess puzzle while blindfolded, or would be taking on the Controlled Data's computer in St. Paul Minnesota, while sitting in the shade of the sycamore trees in the Paul Masson Vineyard in the mountains of Saratoga.

Whatever the case, the day after the World Chess Championship began (and while George Koltanowski was giving his performance in Saratoga), the Zodiac Killer mailed a letter to the San Francisco Chronicle on July 19th 1978, stating "But maybe you play chess with me. I have several cheap sets in closets all over. I have my name on the bottom of the lid with the scotch tape....My tape is waiting for me all over California". Clearly this was no coincidence, especially when you consider that this Zodiac letter was probably composed on July 18th 1978, the day the World Chess Championship began, the day that George Koltanowski gave a talk in Saratoga about the opening chess match between Anatoly Karpov and Viktor Korchnoi, and the same day that the Santa Cruz Sentinel (and numerous others) reported on this very same chess game, with a Zodiac Killer newspaper article right next to it, entitled "SF Chief Says Inspector Didn't Write Zodiac Letter". A newspaper article on July 18th 1978 about David Toschi who had been demoted to pawn detail, followed by a Zodiac letter on July 19th 1978 inviting the former inspector to "maybe play chess". 

In the Zodiac Killer letter postmarked July 19th 1978, it stated "I am the ZODIAC and I am in control of all things. I am going to tell you a secret. I like friction tape. I like to have it around in case I need to truss someone up in a hurry....I have my real name on a small metallic tape. You see, while you have it in your possession, I want you to know it belongs to me and you think I may have left it accidentally. I am athletic. It could be swim fins, or a piece of scuba gear. But maybe you play chess with me. I have several cheap sets in closets all over. I have my name on the bottom of the lid with the scotch tape....My tape is waiting for me all over California. Do you know me? I am the ZODIAC and I am in control". You will notice that the letter mentions Scotch Tape, invented in 1930 by 3M engineer Richard Drew in St. Paul, Minnesota (and manufactured there). The very location of the computer being challenged by George Koltanowski at the time the Zodiac Killer letter arrived at the San Francisco Chronicle, mentioning chess. Designed as a moisture-proof sealant for food packaging, it quickly became popular during the Great Depression for repairing household items. The Zodiac Killer informed us that he had his name "on the bottom of the lid with the scotch tape", leading many to believe that the Zodiac Killer's name may have been "Paul", contained within the location of St. Paul, Minnesota. 

Is it possible that the Zodiac Killer attended this chess tournament at the Paul Masson Vineyard in Saratoga as a competitor or spectator? This location is situated 2 miles (by crow) from where Kathy Bilek (18) was stabbed to death on April 11th 1971 in the woods of Villa Montalvo, whom the Zodiac Killer claimed responsibility for on July 13th 1971 by mailing the "Monticello" card, stating "Near Monticello Shought Victims 21 ...... In The Woods Dies April". Saratoga is close to San Jose, which has consistently featured in the Zodiac Killer story.

The company (3M) that produced Scotch Tape also invented Post-it Notes (sticky notes) in 1974. George Koltanowski famously used sticky notes (or small pieces of paper) as part of his demonstration of the "Knight's Tour" during his exhibitions. During his demonstration of the knight's tour, George Koltanowski would ask audience members to call out random words, names, or numbers, which were written on sticky notes and placed on each of the 64 squares of a large demo chessboard. He was then blindfolded, and another person would select a random square. Starting from that square, Koltanowski would recite the text on the note and continue to call out the contents of every other square in sequence, without error, following the path of a knight's tour until all 64 notes were removed. A knight's move that was required to solve the Zodiac Killer's 340 cipher, mailed to the San Francisco Chronicle where George Koltanowski had a chess column. The audience members sometimes had their names placed on sticky notes, not unlike the Zodiac Killer, who claimed his name was on Scotch Tape all over California. 

The Scotch Tape letter upon first reading makes little sense, but everything contained within Zodiac communications usually have their foundation in Bay Area newspapers, which the Zodiac Killer borrowed from and inserted, often cryptically, into his writings to the San Francisco Chronicle (and occasionally others). Whether this extended to the 340 cipher through an interest in chess, is open to question.    

FURTHER READING: A PAWN IN A GAME OF DEATH

It is almost impossible to forge a solid connection to the Zodiac Killer from the Riverside communications, despite some interesting parallels, such as the trinity of letters mailed to the Riverside Press-Enterprise, Riverside Police and Joseph Bates in 1967 (and the Confession letters), foreshadowing a similar methodology in the Bay Area of northern California, with communications to the Vallejo Times-Herald, San Francisco Chronicle and San Francisco Examiner. Both sets of letters possibly containing the letter "Z" and giving us a common thread to "The Most Dangerous Game" and Count Zaroff of yesteryear.

​The two Confession letters mailed in 1966 stated "I am not sick. I am insane. But that will not stop the game" and "I am stalking your girls now", and offered us a message reminiscent of the flawed character traits of Count Zaroff and Erich Kreiger (A Game of Death, 1945), who were insane, heartless, and psychopathic men with a thirst for stalking and hunting human beings. The question being, were the Confession and Bates letters a precursor to the arrival of Zodiac on July 31st 1969, who boasted that "it was more fun than killing wild game in the forest because man is the most dangerous animal of all". A "game" that was played in Riverside, Benicia and Vallejo - and would continue into the middle half of 1971, with the "Z" of Count Zaroff seemingly never far away. 

The Zodiac Killer should have been unaware of the Bates letters in 1969 (and what some believe is a stylized "Z" signature), had he not been the author, so it may be noteworthy to point out the possibility of the 408 cipher being signed with the identity of "Z", present on the ring of Count Zaroff in "The Most Dangerous Game" (1932). The Zodiac Killer used the exact phrase uttered by Merian C. Cooper, the associate producer of the movie, when interviewed just prior to its release, by encoding the the words "man is the most dangerous animal of all" in his 408 cipher. So would it really be that unusual if he also encoded the letter "Z", taken from the main protagonist and big game hunter, in a story of unbridled madness? 

It is fairly clear that the Zodiac Killer made an error when taking his draft 408 message and separating it into three parts, missing out one complete row of seventeen characters when moving between his letters to the San Francisco Examiner and San Francisco Chronicle. The message on rows 17, 18 and 19 all began with the plaintext letter "E", so it is perfectly easy to comprehend how the Zodiac Killer accidentally skipped one row and left himself with 18 unwanted spaces rather than one. This is where I believe he intended to place his identity of "Z", synonymous with the villainous Russian aristocrat, Count Zaroff, who also enjoyed killing people for nothing more than entertainment.

On the left is the message he likely intended to encode, leaving us with "AFTERLIFEE" at the end of his cipher. The Zodiac Killer placed a dotted circle in the 391st position to give us the extra "E" in afterlife, so one could argue this was supposed to originally complete the 408 cipher and be the signature we had to work out. Only when the cipher key was understood, did it become apparent that the initial ciphertext character to represent the letter "E" in his cipher key was "Z". Whether this was the Zodiac's thinking is unclear, but the evidence is fairly compelling that he never intended to leave us a garbled message of 18 characters, that nobody has come close to explaining in over half a century. The massively interpretative quality of 18 jumbled letters should absolutely tell you that no definitive solution could ever be satisfactorily proven to be the correct answer - and as such - was never the intention of the Bay Area murderer, who was certainly no dummy. But the letter "Z" was, without doubt, an identity he would employ in later communications, with one additional key ingredient always present. This would show itself on October 27th 1970, when the Zodiac Killer selected his Halloween card with a specific message.       

The author of the typed Confession letters on November 29th 1966 claimed their insanity would "not stop the game" of murder, with the Zodiac Killer on July 31st 1969 bragging that killing people was "more fun than killing wild game in the forest", by taking his cues from "The Most Dangerous Game" (1932), where the letter "Z" both down south and up north, may have been the signature. Then came the Halloween card on October 27th 1970, in which the Zodiac Killer addressed Paul Avery and selected a card with the message "But, then why spoil our game", while visibly adding the letter "Z" to a communication for the first time, just 20 days prior to the Paul Avery article on November 16th 1970, informing us of the Bates letters from 1967 with a Z-like signature. Seven months later, another letter and cryptogram of 148 characters would be mailed to the San Francisco Chronicle newspaper, carrying the message "man is the most prized game", taken straight from the playbook of General Zaroff and "The Most Dangerous Game". This letter was signed with a sun cross, the ciphertext character representing the letter "Z" in the cipher. The Zodiac Killer only utilized the word "game" in three communications, on July 31st 1969, October 27th 1970 and May of 1971, with the letter "Z" plausibly embedded in the coding of two, and clearly present in a third. If the Zodiac Killer authored the communications in Riverside, it's possible he added a fourth - and may very well have been correct when he stated his insanity would "not stop the game", as many investigators had the misfortune to find out. 

Richard Connell book, The Most Dangerous Game (1924): "My dear fellow," said the general, "have I not told you I always mean what I say about hunting? This is really an inspiration. I drink to a foeman worthy of my steel--at last." The general raised his glass, but Rainsford sat staring at him. "You'll find this game worth playing," the general said enthusiastically." Your brain against mine. Your woodcraft against mine. Your strength and stamina against mine. Outdoor chess! And the stake is not without value, eh?"