The Halloween card alliance to the Lake Berryessa attack formed "by knife", in conjunction with the knothole in the tree and the wording "peek-a-boo you are doomed", should leave little doubt the two are connected. There is also a strong connection from the Halloween card to both the December 7th 1969 and December 16th 1969 Fairfield letters, and in turn, all three to the 340 cipher. The December 16th 1969 Fairfield letter contained a crude arrangement of 9 symbols (10 including the large crosshairs), impossible to decode as it stood. The four crosshairs around a larger version looked extremely similar to the Halloween card arrangement - and bearing in mind the December 16th 1969 Fairfield letter was accompanied by a crude drawing of a knife entitled "The Bleeding Knife of Zodiac", it seemed likely the short code may have been Zodiac's initial attempt at revealing the hidden words on the car door of Bryan Hartnell's 1956 white Karmann Ghia.
Thanks to Tahoe 27's excellent find linking the Halloween card to a Tim Holt comic book, depicting Death By Fire, Death By Gun, Death By Rope and Death By Knife, it seemed obligatory to consider the connection from the Halloween card "by knife" to the "The Bleeding Knife of Zodiac", along with the similarity in design between the Paradice/Slaves arrangement and the four crosshairs circling a larger version. The four methods of death represented by the four crosshairs or targets.
The December 7th letter opened with the words "I just need help", reminiscent of the December 20th 1969 Melvin Belli letter, and followed by "I will kill again, so expect it any time the will be a cop", similar in fashion to the April 20th 1970 letter, where Zodiac stated "But there is more glory in killing a cop than a cid because a cop can shoot back". The December 16th letter opened with the same two words as the December 7th letter, stating "I just want to tell you", almost certainly tying them together - along with the handwriting and crude coding. Bearing in mind neither of the Fairfield letters appeared in the newspapers, it appears that Zodiac either just accidentally mirrored these communications with the subsequent December 20th 1969 and April 20th 1970 letters, or he was the author of all four.
Was the Paradice, Slaves, By Fire, By Gun, By Rope and By Knife configuration a representation of the number of murders he was intending to secure for the afterlife. Paradice, Slaves, By Fire, By Gun, By Rope and By Knife oddly contained 38 letters. The character count on the December 7th 1969 Fairfield letter code was 38. The intended victims on the December 16th 1969 Fairfield letter was 38. The 340 'Dripping Pen' card, eerily similar to the "Bleeding Knife of Zodiac" depiction, opened with the wording "Sorry I haven't written, but I just washed my pen." This phrase again contained the words "I just", but also an alphanumeric count of 38. The April 20th 1970 letter, stating "I hope you have fun trying to figgure out who I killed" had a code that added up to 38 when the alphabet was placed alongside it.
And finally, the Exorcist letter (widely believed to be the final offering by the Zodiac Killer) claimed 37 victims, but promising to kill again if his demands were not met. Probably coincidence, but curious nonetheless.
The 'Death By Fire, Death By Gun, Death By Rope and Death By Knife' agenda could have been formed in the mind of the killer as early as the beginning of his campaign of terror, when he mailed the 408 cipher, stating "the best part of it is that when I die I will be reborn in paradise and all the (people) I have killed will become my slaves. I will not give you my name because you will try to slow down or stop my collecting of slaves for my afterlife". The same paradice and slaves permeating through much of his correspondence, and possibly the 340 cipher also. "By knife" an integral part of both Fairfield letters and the Halloween card, to hopefully lay any doubts to rest, of his involvement and knowledge of the writing at the foot of the message on Bryan Hartnell's car door.