Two days after the Kathleen Johns Letter, on July 26th 1970, the Zodiac mailed another letter in which he boasted about the ways he would like to torture his slaves in paradise. See pages of the Little List below right.
A possible solution to the June 26th 1970 Button letter code and Phillips 66 Map may have been provided by the Zodiac Killer within the two crosshairs accompanying the Little List letter. The solution is constrained by the land mass of San Francisco, in which only 4 radians and magnetic north, and five inches can possibly fall over it. The complete answer can be found below, or in the article The Answer to the Mount Diablo Code.
The first section of the Little List letter pulls lines from the Gilbert and Sullivan comic opera, The Mikado (A more humane Mikado), where the author uses the words billiard along with crooked cues and twisted shoes. This correspondence goes on to paraphrase Gilbert and Sullivan's Mikado Act One Part 5a As some day it may happen, performed by Ko-Ko, as did the future correspondence of the Exorcist Letter in 1974, when reciting Tit-Willow from Ko-Ko's On a tree by a river, part of Act Two.
The writing at the bottom of the Little List letter reads:
"P.S. The Mt. Diablo code concerns Radians + # inches along the radians".
See the Button letter and Phillips 66 Road Map, mailed to the San Francisco Chronicle on June 26th 1970, exactly one month prior to the Little List letter, with the crosshairs of Zodiac positioned over Mount Diablo.
San Francisco Chronicle news article - Gilbert and Sullivan Clue to Zodiac
This is the Zodiac speaking
Being that you will not wear some nice buttons, how about wearing some nasty buttons.
Or any kind of buttons that you can think up. If you do not wear any type of buttons, I shall (on top of everything else) torture all 13 of my slaves that I have waiting for me in Paradice. Some I shall tie over ant hills and watch them scream + twich and sqwirm. Others shall have pine splinters driven under their nails + then burned. Others shall be placed in cages + fed salt beef untill they are gorged then I shall listen to their pleass for water and I shall laugh at them. Others will hang by their thumbs + burn in the sun then I will rub them down with deep heat to warm them up. Others I shall skin them alive + let them run around screaming. And all billiard players I shall have them play in a darkened dungen cell with crooked cues + Twisted Shoes. Yes I shall have great fun inflicting the most delicious of pain to my slaves.
As some day it may hapen that a victom must be found. I've got a little list. I've got a little list, of society offenders who might well be underground who would never be missed who would never be missed. There is the pestulentual nucences who whrite for autographs, all people who have flabby hands and irritating laughs. All children who are up in dates and implore you with im platt. All people who are shakeing hands shake hands like that. And all third persons who with unspoiling take thoes who insist. They'd none of them be missed. They'd none of them be missed. There's a banjo seranader and the others of his race and the piano orginast I got him on the list. All people who eat pepermint and phomphit in your face, they would never be missed They would never be missed And the Idiout who phraises with inthusastic tone of centuries but this and every country but his own. And the lady from the provences who dress like a guy who doesn't cry and the singurly abnormily the girl who never kissed. I don't think she would be missed Im shure she wouldn't be missed. And that nice impriest that is rather rife the judicial hummerest I've got him on the list. All funny fellows, commic men and clowns of private life. They'd none of them be missed. They'd none of them be missed. And uncompromising kind such as wachmacallit, thingmebob, and like wise, well - - nevermind, and tut tut tut tut, and whashisname, and you know who, but the task of filling up the blanks I rather leave up to you. But it really doesn't matter whom you place upon the list, for none of them be missed, none of them be missed.
PS. The Mt. Diablo Code concerns Radians & # inches along the radians
On April 20th 1970 the Zodiac Killer mailed a letter to the San Francisco Chronicle with three key components.  The Zodiac Killer stated "I hope you do not think that I was the one who wiped out that blue meannie with a bomb at the cop station. Even though I talked about killing school children with one. It just wouldn't doo to move in on someone else's teritory". The Zodiac Killer was referencing the murder of San Francisco police officer Brian McDonnell on February 16th 1970, who suffered devastating and sadly fatal injuries two months earlier, when a bomb packed with fence staples exploded on an outside ledge of Park Police Station in the Upper Haight neighborhood. He continued  "But there is more glory in killing a cop than a cid because a cop can shoot back".  The Bay Area murderer also included a bus bomb diagram, with a school bus traveling south in the morning and the sun rising to the east. The bomb was positioned on a steep embankment.
It therefore shouldn't be any surprise that his next letter on June 26th 1970 contained all three of these components in its design:  He claimed he had set a bomb by Ingleside Police Station,  claimed his "glory" by insinuating his involvement in the shooting death of Officer Richard Radetich and  his bus bomb diagram matched perfectly the bomb to be set by Ingleside Police Station.
In the June 26th 1970 Button letter he stated "The Map coupled with the code will tell you where the bomb is set*. It is clear the bomb was to be set in San Francisco, the target for the majority of his communications and ire toward police. The code provided the inches (5 to be exact) and the map was to provide the radians, to be set at Mag. N (magnetic north) from his zero point clockwise. This was plainly evident when he provided the extra two clues in the following Little List letter on July 26th 1970.
The first clue was the first set of crosshairs, where he placed the bold, black circle at 4 radians and magnetic north (246 degrees} around the circumference. This was to be placed over the Mount Diablo map of San Francisco & Vicinity, centered over Mount Diablo to give us the radian value. The second clue was therefore the solution to the 32 symbol cipher containing the inches - where he placed the hashtag and arrow for the number 5 to be inserted. Therefore, we had to travel approximately 5 inches along a line subtended at 246 degrees from Mount Diablo. This is why he used the phrase "The map coupled with the code will tell you where the bomb is set". The image on the right overlayed on the map gave us the radians or degrees, coupled with the code which only contained the inches value.
The bomb was to be set close to Ingleside Police Station, which to a high degree of accuracy fell extremely close to 4 radians and magnetic north, and five inches from Mount Diablo. The inches subtended from Mount Diablo at 246 degrees couldn't be more or less than 5, because a bomb (set at 4 or 6 inches) would have fell into the sea on the east and west side of San Francisco. The bold, black circle (above right) with SFPD next to it, should be evidence enough that the target was squarely focused on the San Francisco Police Department. Not withstanding the fact, that the geography of San Francisco is such, that 4 radians and 5 inches are the only measurements capable of falling within San Francisco. The Zodiac Killer gave us a zero pointer (facing north) on his Mount Diablo map, to be set to Magnetic North of 17 degrees, with a clock face around the circumference. This clearly indicated that the radians were to be measured in a clockwise fashion - and therefore no surprise that the bold, black circle was positioned at exactly 4 radians and magnetic north (see below).
I have placed the number 5 into place, giving us the answer to the code of "Radians and 5 inches along the radians" (which is 32 letters in length). The positioning of the bomb had to meet certain criteria. With a high degree of accuracy, Ingleside Police Station was situated 4 radians and magnetic north, and 5 inches from Mount Diablo.
Even a line subtended from Mount Diablo passing over the neighborhood of Ingleside in San Francisco, when considered alongside the addition of SFPD=0, should have told us that the target for the bomb was likely the San Francisco Police Department in Ingleside. No other police station in San Francisco comes close to 4 radians and magnetic north, and 5 inches along the radians. The next task was looking for the exact location for the bomb to be buried. It most likely would have been set in the vicinity of the police station, just distant enough not to be observed.
The bus bomb diagram in the April 20th 1970 letter (shown above) depicted a school bus traveling from north to south in the morning, next to a steep embankment, with the sun rising to the east. This location is just 230 feet from Ingleside Police Station. School buses traveling from the northern and eastern edge of San Francisco to St.Thomas More Catholic School would travel the Southern Freeway, passing only 210 feet from Ingleside Police Station. From this location the bus would be traveling south in the morning to begin classes for the day, the sun would be rising from the east, and this location has a steep embankment mirroring exactly his bus bomb diagram of April 20th 1970. This portion of the Southern Freeway is flanked by the parking lot at the City College of San Francisco. This parking lot is the ideal location to access the embankment without parking on the freeway.
Every criteria has now been met. The school bus, the route of the school bus on a freeway, the southerly direction, the steep embankment, the target of SFPD (San Francisco Police Department) and the 4 radians and magnetic north, coupled with the code of 5 inches. The 32 character code wasn't the complete solution, because the Zodiac Killer explicitly stated "the map coupled with the code will tell you where the bomb is set". The code contained the inches by way of "Radians and 5 inches along the radians", to be coupled with the map which contained the 4 radians and Mag. N (246.2 degrees). A bomb set here would have had maximum impact, striking a school bus as Zodiac had promised, but more importantly, right under the noses of the San Francisco Police Department for acute embarrassment.
The location of a proposed bomb on the Southern Freeway by Ingleside Police Station, matches both the Mount Diablo map and code of 4 radians set to magnetic north, and 5 inches along the 4th radian. The location also matches perfectly the bus bomb diagram within the April 20th 1970 letter.
The Ingleside Police District encompasses the area south of Cesar Chavez Street to the San Mateo County line and west from Highway 101 to Faxon Avenue. It is home to the San Francisco City College campus and many diverse residential neighborhoods including Sherwood Forest, Mt. Davidson, Glen Park-Sunnyside, Diamond Heights, Bernal Heights, Upper Noe, Mission Corridor, the Excelsior and St. Francis Wood, St. Mary's Park, Visitacion Valley, Precita Park, Cayuga, Outer Mission and Holly Park.
This card was mailed on October 5th 1970, one month after the disappearance of Donna Lass, who vanished after her shift at the Sahara Tahoe Hotel, Nevada on Sept 6th 1970. There were 13 punch holes on the right side of this postcard, thought to denote the amount of Zodiac victims, and a pasted red cross of thin paperat its center. Initially this card was considered debatable Zodiac correspondence, but is now believed to be genuine after comparison to other confirmed styles of contact mailed from the killer. The image below right has been edited, with the text flipped. Extra reading on the 13 Hole postcard
The answer to the 32-Symbol cipher was given to us by the Zodiac Killer after just 30 days when he mailed the Little List letter on July 26th 1970. He gave us part of the answer when he wrote PS. The Mt. Diablo Code concerns Radians & # inches along the radians (all we had to do was add the number 5 where he placed the arrow and hashtag}. The code solution identified was "estimate: four radians and five inches". The 32 character code wasn't the complete solution, because the Zodiac Killer explicitly stated "the map coupled with the code will tell you where the bomb is set", which included magnetic north of 17 degrees in 1970. The code contained "four radians and five inches", to be coupled with the map which incorporated the magnetic north, giving us a total of 246.2 degrees. See
The Mount Diablo Map & Code Solution
CLICK LETTERS TO MAGNIFY
1963 June 4th- Domingos/Edwards Murders
1966 October 30th-The Cheri Jo Bates Murder
1966 November 29th -The Confession Letter
1966 December- The Riverside Desktop Poem
1967 April 30th- Bates Had to Die Letter
1968 December 20th- Lake Herman Murders
1969 July 4th-The Blue Rock Springs Attack
1969 July 31st- Vallejo Times-Herald Letter and
408 Cipher Page1 (solved)
1969 July 31st- Examiner Letter and
408 Cipher Page 2 (solved)
1969 July 31st - Chronicle Letter and
408 Cipher Page 3 (87% solved)
1969 July 31st-The Complete 408 Cipher
1969 August 4th- Debut of Zodiac Letter
1969 August 10th- Concerned Citizen Card
1969 September 27th- Lake Berryessa Attack
1969 October 7th- The Good Citizen Letter
1969 October 11th- Presidio Heights Murder
1969 October 13th-The Paul Stine Letters
1969 October 22nd-Call to Chat Show
1969 November 8th- Dripping Pen Card and
340 Cipher (solved in 2020)
1969 November 9th-The Bus Bomb Letter
1969 November 19th- The Riddler Notes
1969 November 21st-The San Jose Code Letter
1969 November 28th- Betsy Aardsma Murder
1969 December 7th-The Fairfield Letter
1969 December 10th- Forecast for Cancer
1969 December 11th- Forecast for Leo
1969 December 16th-The Fairfield Letter
1969 December 20th-The Melvin Belli Letter
1970 Feb 21st- The Hood and Garcia Murders
1970 March 22nd-The Modesto Attack
1970 April 15th-The Robert Salem Murder
1970 April 20th-"My Name is" Letter,
Cipher 3 and Bus Bomb Diagram
1970 April 28th-The Dragon Card
1970 June 26th-The Button Letter
Cipher 4 (solved in 2019) and Map
1970 July 4th-The Sleeping Bag Murders
1970 July 24th-The Kathleen Johns Letter
1970 July 26th-The Little List Letter
1970 Sept 6th-The Donna Lass Disappearance
1970 October 5th-13 Hole Postcard
1970 October 17th- "You Are Next" Postcard
1970 October 27th-The Halloween Card
1971 March 13th-The Los Angeles Letter
1971 March 22nd-The Pines Card
1971 Unknown- The 148 Character Cipher
1971 July 13th-The Monticello Card
1973 August 1st- The Albany Letter
1974 January 29th-The Exorcist Letter
1974 February 3rd-The SLA Letter
1974 May 8th-The Citizen Card
1974 July 8th-The Red Phantom Letter
1974 December 27th- Christmas Card
1975 November 3rd- The Belmont Letter
1978 April 24th-The 1978 Letter
1978 May 2nd- The Channel Nine Letter
1978 July 19th- The Scotch Tape Letter
1981 March 8th- The Peachtree Letter
1986 May 6th- The Freeway Letter
1987 October 28th- The 1987 Letter
1988 February 1st- The McDonald's Letter
1988 February 8th- The McDonald's Letter 
1990 September 25th- The Celebrity Cypher
1990 December-American Greetings Card
2001 January 10th- Happy New Year Card
Unknown DMV Letter (possibly November 1971)
The following postcard was postmarked October 17th 1970, but dated as October 12, 1970 on the correspondence. It arrived just under two weeks after the 13 Hole postcard, claiming to be from the Zodiac Killer.