In 2012, Mike Morford of Zodiac Killer Net interviewed Phil Sins, who stated that despite going to the Riverside Police Department with Paul Avery "where the letters & desktop were discussed or shown", he himself did not get to see them. Therefore, he had no knowledge regarding the contents of the Bates' letters when he drew his original conclusions between the Zodiac and Riverside cases. And neither, have we yet proved, did the Zodiac Killer. This is why the Halloween card and the Zodiac Killer's previous letter, the Little List letter mailed on July 26th 1970, are so curious. If not responsible for any of the Riverside activity, the Zodiac Killer could have been aware of the Cheri Jo Bates case and the Confession letter when he mailed both the Little List letter and Halloween card, because the Inside Detective magazine published on January, 1969 featured both. However, he wouldn't have been aware of the three Bates' letters (had he not been the author).
The whole premise of the Halloween card cover was seemingly chosen on the understanding that when the card was opened it would reveal his name. One notable feature of the card was the prominence of the word "By", followed by the four methods of death. The Zodiac Killer had written "By Fire, By Gun, By Rope and By Knife". The 1966 Confession letter also tempted us with a name, when the author began his typewritten letters with "The Confession", followed by the word "By" and several underscores. The word "By" playing a significant role in both communications. We also have the Halloween theme running through the 1970 card and the Riverside murder of Cheri Jo Bates, whose lifeless body was discovered on Halloween morning. But it's the addition of the "Z" symbol (meant to infer "Zodiac") placed at the foot of the Halloween card that conjures the most interest. The Zodiac Killer had mailed 15 confirmed communications prior to October 27th 1970 (probably in excess of 20), yet he had never once used the letter "Z" to sign off a communication. In fact, no communication subsequent to October 27th 1970, verified or otherwise, ever signed off with the letter "Z".
The Zodiac Killer may have done exactly the same thing on November 8th 1969 when he claimed seven victims within the Dripping Pen card, forcing investigators to draw the conclusion he was responsible for the murder of Kathie Snoozy and Debra Furlong in San Jose on August 3rd 1969 by the addition of "Aug" in his chronological list of months. As he did on March 22nd 1971, mailing us another cryptic card, this time encouraging investigators to connect him to the Donna Lass disappearance on September 6th 1970 by the addition of "Lake Tahoe areas" and "Sought Victim 12". Once Paul Avery had connected the Riverside and Zodiac murders together, the Zodiac Killer then cemented that notion by giving "credit for stumbling across his Riverside activity". However, not once did he ever categorically claim or prove these murders were his. He only let investigators draw such a conclusion.