The Halloween card was mailed by the Zodiac Killer on October 27th 1970, it was addressed to Paul Avery, it referenced Halloween and it contained the letter "Z". Fifteen days later, Paul Avery is reporting on Cheri Jo Bates being found murdered on Halloween morning, with the Bates' letters signed with a letter "Z". Paul Avery's article was likely triggered by somebody (Phil Sins) noticing similarities between the Halloween card and the murder of Cheri Jo Bates, but what were the chances of somebody recognizing something in the Halloween card in respect to the murder of Cheri Jo Bates, pursuing this angle, and subsequently finding a "Z" like signature that mirrored the Halloween card? (used for the first time by Zodiac). What are the chances of Zodiac marrying Halloween and the pseudonym "Z" together, which mirrored the discovery of a murder victim on Halloween morning, which later had accompanying letters signed with a "Z" style signature? The Halloween card coming just three months after the Zodiac Killer mirrored the Cheri Jo Bates Confession letter of "She squirmed and shook as I choaked her, and her lips twiched. She let out a scream once": when he wrote in the Little List letter on July 26th 1970 "Some I shall tie over ant hills and watch them scream + twich and sqwirm".
In a time span of just three months, the Zodiac Killer had combined scream, twich, squirm, Halloween and a "Z" signature, possibly merging crucial details from the Confession and Bates letters together. Four of these details were already available in the January 1969 edition of the Inside Detective magazine, but the "Z" signature was not. If the signature on two of the Bates' letters was really meant to be the letter "Z", then the fact that the Zodiac Killer broke with tradition and used the pseudonym "Z" within a Halloween card for the first time, could be argued as carrying some significance.