For those of you that research letter writing serial killers and know about the Zodiac Halloween Card, you might like to try Chapters 12 and 13 of the book on that site which is up for free on the pulldown menu. All of it is there completely for free, the entire book start to finish if you wish but 12 and 13 are most relevant.
There is a discussion of the findings on https://www.tapatalk.com/groups/theskeletonsaredancing/ with more research since the book came out.
Unlike other attempts to fit a POI into a code this guy was found after a particular puzzle possible solution involving TIM HOLT comics was explored more.
It is a bit of a rabbit hole, but I think some sleuths here might be interested in it.
Happy New Year 2020!