'For 35 years starting in 1965, the San Francisco Chronicle and Examiner operated under a Joint Operating Agreement whereby the Chronicle published a morning paper and the Examiner published in the afternoon. The Examiner published the Sunday paper's news sections and glossy magazine, and the Chronicle contributed the features.' Wikipedia. 
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On August 4th 1969 the San Francisco Examiner received the 'Debut of Zodiac' letter, in which the killer revealed his pseudonym for the first time. This letter was not planned, as it was only mailed by the killer in response to an article released in the newspaper over the weekend, in which Vallejo Police Chief Jack E. Stiltz implored the murderer of three to supply more details about his crimes, to effectively prove that the author of the July 31st 1969 letters was in fact the killer. The Zodiac duly obliged. The first newspaper article was released by the San Francisco Chronicle on Saturday August 2nd 1969. Jack E. Stiltz requested the writer to send another letter "with more facts to prove it." The second newspaper article followed on Sunday August 3rd 1969 in the San Francisco Examiner. Jack E. Stiltz was 'still not convinced the letters and codes were written by the actual killer.' The police chief 'urged the writer to send more letters, with more facts to prove his connections to the crimes.'  The 'Debut of Zodiac' letter was hand delivered to the San Francisco Examiner the following morning, giving rise to the notion he is responding to the Sunday article in particular. This may be important if we analyze the nature of the communication and the actual paper it is written on. 

The Zodiac committed four attacks we know of. The first two at Lake Herman Road and Blue Rock Springs were perpetrated on a Friday in the hour preceding midnight. The Lake Berryessa and Presidio Heights attacks were perpetrated earlier, both on Saturday. This fed into the belief that Zodiac had a regular Monday to Friday, 9-5 job, with his Friday attacks likely committed closer to home due to time constraints, whereas on Saturday he could venture further afield, as it was one of his rest days. This may have come into play when the Zodiac delivered his August 4th 1969 letter.

​The Zodiac had just embarked on his letter writing campaign, when he mailed the 408 cipher on July 31st 1969, split into three sections of 136 characters and each mailed to three newspapers, the San Francisco Chronicle, San Francisco Examiner and Vallejo Times-Herald, all of which decided to publish. When Jack E Stiltz questioned the validity of the letter writer, Zodiac was eager to reply, shown by the fact his response was received on Monday morning, the day after the August 3rd 1969 Sunday Examiner article. He must have hurriedly drafted a response in order that his letter was published in the afternoon edition, but with one key difference.

The Zodiac Killer commonly used Eaton brand paper in his communications with the newspapers, but the 'Debut of Zodiac' letter was a notable change, using Woolworth's Fifth Avenue paper, suggesting he was thrown out of his normal routine. No doubt he kept a supply of Eaton paper at his residence, which he reverted back to after the 'Debut of Zodiac' letter. It seemed like the Woolworth's Fifth Avenue brand paper was a rushed buy, when he had no easy access to his home location, yet was still in a location to readily purchase the Sunday Examiner newspaper.. Here is one possible scenario, but obviously not the only one.

"The Zodiac Killer has just completed his 408 cipher and the weekend is approaching. He decides to use his Saturday and Sunday off work to relax and bask in his new found notoriety. If he is a resident of Vallejo or Benicia, then a thirty mile weekend trip away to San Francisco, may seem like a justified reward to enjoy his passion of theater in the Union Square and Tenderloin District. San Francisco may be an area he is already familiar with, if he works there. Then Jack E Stiltz puts a fly in the ointment by issuing a doubt that the killer and letter writer is the same person. The Zodiac Killer enjoying a cup of coffee over breakfast in his hotel getaway, picks up his paper and nearly chokes when he reads the article. Young and impulsive, his need to respond is immediate before Monday's edition. He finishes breakfast in his theater district hotel and heads to the nearest stationers to purchase some paper and a felt tip pen."  

"The first letter in which he christened himself "Zodiac" carried a different watermark than three earlier letters (Monarch-cut bond, imprinted with an Eaton watermark). The new watermark was fifth avenue, an imprint of Frank Winfield Woolworth's national chain. A huge Woolworth's stood just a block away from the Chronicle, at the cable car turntable at Fifth and Market and Powell. In the basement, next to the goldfish, Woolworth's sold blue felt-tip pens and paper exactly like Zodiac used. What if he bought his paper and blue felt-tip pens there?"  Robert Graysmith. 
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His Eaton paper was at his residence, but he is on a weekend trip. The easy option is to purchase a temporary pad of paper from the nearest location and Woolworth's would have been the obvious choice. It is in the heart of the theater district, just 10 minutes walking distance from Union Square and the Westin St Francis Hotel, the location where Zodiac was believed to have flagged down Paul Stine just two months later on October 11th 1969. Robert Graysmith highlights the Westin St Francis Hotel at Union Square, along with the Pinecrest Restaurant in his book 'Zodiac', whereas the article in the San Francisco Examiner on October 23rd 1969 stated "The search also took police to Nob Hill, where the Fairmont Hotel became involved again in the manhunt. This was because: Cabbie Paul Stine, 29, the Zodiac's latest victim is believed to have picked up his fare on a street near the famed hotel. A waitress in the hotel candy shop-fountain told police she served coffee during the television show (Jim Dunbar/Melvin Belli) to a man who resembled composite drawings of Zodiac. She saved the cup and saucer for a fingerprint check."
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If Zodiac was a visitor to one of these hotels, then the footnote in his 'Debut of Zodiac' letter declaring "No address" may carry some irony. The Zodiac Killer would later send two further missives, the 'Little List' and 'Exorcist' letter, in which he featured the Gilbert and Sullivan satirical comedy, The Mikado, which played in the San Francisco theater district during this period. This location of Woolworth's, the theater district, the offices of the San Francisco Chronicle and San Francisco Examiner and the pick-up-point of Paul Stine on October 11th 1969, certainly center the focus on this small area of San Francisco. The San Francisco Chronicle at 901 Mission Street/corner of 5th, and San Francisco Examiner at 835 Market Street are key locations.   

The murder of Paul Stine on Saturday October 11th 1969 was thought to have originated from a pick-up-point, either at the Westin St Francis Hotel or the intersection of Mason and Geary Streets. The time would have been approximately 9.30-9.40 pm - so a theater production ending at this time may have provided a connection. Was the Zodiac Killer again on a weekend away, enjoying his passion for the arts, only this time murder was on the menu? However, this murder was a little different. If the killer, as suggested earlier, was not a resident of San Francisco, but a guest at a hotel somewhere in the theater district, then you would assume this murder was pre-prepared. Just like the 'Debut of Zodiac' letter that was received at the San Francisco Examiner on the immediate Monday to the article, so was the October 13th 1969 Paul Stine letter, containing the swatch of the taxicab driver's shirt. His response to both events was almost immediate, yet this time, knowing his murderous intentions, was prepared with pen and paper in advance.

The Zodiac Killer mailed the 'Exorcist' letter on January 29th 1974 in response to the San Francisco Chronicle article entitled 'Weird Goings on at the Movies,' authored by Paul Avery and published on January 11th 1974, detailing the audience reaction to the recently released Exorcist movie in 1973. The news report was from the Northpoint Theatre, located at 2290 Powell Street, San Francisco. The theater was located 3.4 miles from the Washington and Cherry Street intersection, where Paul Stine was murdered, and only 1.7 miles from the region of Union Square, where the taxicab driver picked up Zodiac.
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If the Zodiac Killer was a regular visitor to the area of San Francisco's theater district, then the Northpoint Theatre, easily accessible from Union Square, may have saw the murderer through its doors at some point, although he appeared notably underwhelmed by The Exorcist, describing it as "the best satirical comedy that I have ever seen,"
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​Does the change in writing material give us an insight into the Zodiac Killer's movements precedent to the 'Debut of Zodiac' letter, or is it simply a case of reading nothing into something.   

The negro male was supposedly never identified or mentioned in the Blue Rock Springs police report. Was the Zodiac lying about this encounter as many people would argue, or was this a genuine encounter by the Zodiac Killer? Here we will run through some of the arguments.   

In the police report it stated a call received at 12.40 am by unknown citizen: "I wish to report a double murder. If you will go one mile east on Columbus Parkway to a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye."

At 12.47 am Mrs Johnson PT&T operator called. The above call was traced to a coin operated telephone at Joe's Union, Tuolumne and Springs Road. The call was traced by Betty Main, whose supervisor would not allow her to give a statement at this time.

The Zodiac Killer called at 12.40 am and the call was traced inside of seven minutes, which is not possible with the technology of 1969. A caller had to be on the line a minimum of 15 minutes for a call to be traced. Although not completely  accurate, this was referenced in the Zodiac (2007) movie, featuring a clip of the Melvin Belli 'call to chat show,' when Belli was instructed "Pacific Telephone says you need to keep him on the line for 15 minutes."

​It is likely the word 'traced' is being used in the loose sense of the word, and they actually 'discovered' the call was made from Springs and Tuolumne. 

The answer may lie in Zodiac's wording at the top of the page where he said "When I hung the phone up the damn thing began to ring & that drew his attention to me + my car".  Betty Main traced the call but was not allowed to give a statement. This could have meant that what the witness had to say contained sensitive material, having been kept under wraps at that particular juncture. If the Zodiac Killer was telling the truth and the phone rang almost immediately after he hung it up, this was not likely a coincidence. Was it possible, that although Zodiac hung up the receiver, the operator didn't and was able to ring back the payphone? This would not identify the specific location of the payphone, but if somebody had picked it up the operator could ask the person on the other end to identify their location, ie; the negro male Zodiac identified as walking by. We know police officers were redirected from the Blue Rock Springs Park to the corner of Springs Road and Tuolumne Street, and it is here the witness to the phone call may have been interviewed. It is equally conceivable that the Zodiac Killer gave the payphone number to the operator when he was patched through to the Vallejo Police Department. Either way, the Springs and Tuolumne payphone was rang back by the operator.
  
​Michael Cole, author of Zodiac Revisited made some interesting points on the Zodiackiller.com forum:
(1) The information from the letter is being given in direct response to a request for additional identity verification. The other information given in the same context appears to be truthful (mostly). Therefore, I believe Z is being honest in the conveyance of this information. 
(2) We know that Z changed his behavior when he made his call to the Napa PD about three months later. Instead of hanging up the phone, he set it down thus avoiding the possibility of a ringback. This behavior modification is consistent with an experienced-based learning regarding the described ringback; not to mention it just sounds like good police procedure to implement a ringback under these circumstances. Therefore, I suspect the ringback did happen as described. Perhaps Nancy will provide some additional insight on this front. I've already asked the question in the other thread.
(3) We know that Mike Mageau described the car as being brown. 
(4) As mentioned previously, Z's statement implies that he knows somebody told police that his car was brown. Although I don't have the reference, this clearly implies that the detail was published somewhere.
Given all of the above, I suspect that Z honestly believed that the described witness was the person who told police about his car. He apparently, quite reasonably, believed that MM could not have ascertained the color of his car. This left the witness as the most probable source. The fact that the witness was never identified seems to imply that he chose not to come forward or that he was unaware of law enforcement's desire to talk to him."        

The most likely answer is ring-back as Michael Cole suggests, which is how the call was 'traced.' Somebody may have picked up and given the operator the details he had just observed. This was alluded to by Zodiac who stated in the Debut of Zodiac Letter "The man who told police that my car was brown was a negro about 40-45 rather shabbly dressed". It is likely this information was withheld from the police report - and why Betty Main's statement was not made public. Either, as Michael Cole stated "The fact that the witness was never identified seems to imply that he chose not to come forward or that he was unaware of law enforcement's desire to talk to him", or he was located by responding officers, but requested his anonymity be respected and why this information was kept off the record.

But the fact the location of the payphone was 'traced' seems to implicate third party information, in that ring-back alerted somebody to its location. However, that still leaves the question of whether the negro male was interviewed and was able to give any additional information regarding Zodiac's description, that was 'incorporated' into the police report, or did his description simply tally with Michael Mageau's and was therefore not included. Or did he simply get no viable description of the killer or his vehicle at all. Either way, as Michael Cole stated, the Zodiac Killer would not make the same mistake again in Napa on September 27th 1969.       

The Zodiac signed off the Bus Bomb letter mailed on November 9th 1969 with the words "To prove that I am the Zodiac, ask the Vallejo cop about my electric gun sight which I used to start my collecting of slaves."  But to whom was the Zodiac referring. The answer is almost certainly Vallejo Police Chief Jack E. Stiltz, a native Vallejoan who first joined the police force in 1939 and oversaw many changes, including the new station located at 111 Amador Street, before retiring with health issues in 1970. Jack E. Stiltz died in 1974. 

The Zodiac sent three coded segments in three letters on July 31st 1969 to the San Francisco Examiner, San Francisco Chronicle and Vallejo Times-Herald, giving details of his first two crimes to prove he was the actual killer. However, the Zodiac Killer was still doubted, with Police Chief Jack E. Stiltz wanting further proof the killer and letter-writer were one and the same. This can be seen here in this extract from from the Saturday August 2nd 1969 San Francisco Chronicle. He requested the writer send a follow up communication  "with more facts to prove it."  

This story was featured again on Sunday August 3rd 1969 in the San Francisco Examiner (shown here on the left). Jack E. Stiltz was "still not convinced the letters and codes were written by the actual killer". The police chief "urged the writer to send more letters, with more facts to prove his connections to the crimes".

The Zodiac Killer's response was almost immediate, when he hand delivered the Debut of Zodiac letter just one day later, on August 4th 1969 to the San Francisco Examiner, to leave you in no doubt he was replying directly to Jack E. Stiltz. The Zodiac's opening gambit was "In answer to your asking for more details about the good times I have had in Vallejo, I shall be very happy to supply even more material. By the way, are the police having a good time with the code? If not, tell them to cheer up; when they do crack it, they will have me". He went on to describe the Blue Rock Springs Park and Lake Herman Road attacks in much greater detail and effectively informed Jack E. Stiltz about his attached gun sight;  "What I did was tape a small pencel flash light to the barrel of my gun. If you notice, in the center of the beam of light if you aim it at a wall or ceiling you will see a black or darck spot in the center of the circle of light about 3 to 6 inches across. When taped to a gun barrel, the bullet will strike in the center of the black dot in the light. All I had to do was spray them as if it was a water hose; there was no need to use the gun sights. I was not happy to see that I did not get front page coverage". 
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Fast forward to Presidio Heights on October 11th 1969 and Chief Inspector Martin Lee's claims that Zodiac "has made some mistakes." They did not believe that Zodiac could have been hiding in the park. Chief Inspector Martin Lee stated "His boast of being in the area we were searching is a lie. We had the whole area flooded with lights. We had seven police dogs and a large number of patrolmen searching the area tree by tree and bush by bush. The dogs are the best in the country. A mouse couldn't have escaped our attention". They also irked the Zodiac by claiming he had left fingerprints on the interior and exterior of the taxicab, particularly the bloody fingerprints on the driver side door panel.

Zodiac again felt compelled to reply in the November 9th 1969 Bus Bomb letter: "I have grown rather angry with the police for their telling lies about me...... p.s. 2 cops pulled a goof abot 3 min after I left the cab. I was walking down the hill to the park when this cop car pulled up + one of them called me over + asked if I saw anyone acting suspicious or strange in the last 5 to 10 min + I said yes there was this man who was runnig by waveing a gun & the cops peeled rubber + went around the corner as I directed them + I disappeared into the park a block + a half away never to be seen again", and again asked for newspaper coverage similar to the Debut of Zodiac letter by threatening more attacks: "By the way it could be rather messy if you try to bluff me. PS. Be shure to print the part I marked out on page 3 or I shall do my thing. To prove that I am the Zodiac, Ask the Vallejo cop about my electric gun sight which I used to start my collecting of slaves".

The last time the Zodiac Killer was doubted, he supplied extra details of the Blue Rock Springs and Lake Herman Road attacks to Vallejo Police Chief Jack E. Stiltz, including his "small pencel flash light". He would repeat this in the Bus Bomb letter, reminding everybody that he most certainly was the Zodiac Killer. In other words, to not doubt him and "Ask the Vallejo cop about my electric gun sight which I used to start my collecting of slaves". The Vallejo cop he is referring to is Jack E. Stiltz. The Vallejo cop, to which he sent details about his improvised gun sight in the Debut of Zodiac letter only three months earlier. He proved to Jack E. Stiltz that he was both the letter writer and the killer, and again here, in the Bus Bomb letter, he is referring back to this Vallejo cop - and to not doubt him again.