I think Paul received the Halloween card because he was the one who wrote about this card a couple of weeks prior:

Quite possibly…I was actually of the opinion that Avery wrote several pieces about Zodiac prior to then and as we know, Z was normally pretty prompt in replying to events. All of which begs the questions…why wait 5 months to acknowledge the Riverside Connection published by Avery on Nov 16th? And/or if he would have written again at all (until 74) had it not been to respond to it.
You would think that whatever the answer, it could be significant. Perhaps he was away from the area or feared a link might have been made to him and didn’t know how to respond or if he should.
I think also (although Z wrote to Belli in a different context and perhaps later about Count Marco) it may have taken something he took real offence to, in order to single Avery out in that manner. In other words, by definition, he was crediting Avery by default, as being some sort of threat/thorn in his side. That almost had the effect of elevating Avery’s importance, not something you could see Z doing easily IMO.
As to the rest, the questions had to be asked….is it possible Z somehow learned Avery was asking questions about Riverside before publication and/or if there’s something in the Halloween card that Z may have thought, prompted people to look to Riverside.

Do we know how long Avery spent investigating the Riverside Connection before he went public?

I have a different take on the Halloween card than most. Avery was the Chronicle reporter who was writing the vast majority of the stories about the Zodiac. He was the the one person, outside of law enforcement, who was especially knowledgeable about the case. IMO, this is why he was singled out.

The Halloween card was, in relative terms, a desperate plea to be understood. Of course, the precise meaning of the card is open for debate. However, I think most people would agree that even though we do not know the exact meaning of the card, it is likely an important clue that does have some type of substantive meaning. The Zodiac was hoping that Avery would figure it out – use his context knowledge to unravel the mystery.

Unfortunately, everybody concluded the card was a threat against Avery (the detectives, Avery, etc.). Coworkers made "I am not Paul Avery" buttons. Avery began carrying a firearm (for a time). And the mystery woven into the contents of the Halloween card remained, and remains, frustratingly intact.

Some good thoughts & theories guys, thanks for sharing them!

I have a different take on the Halloween card than most. Avery was the Chronicle reporter who was writing the vast majority of the stories about the Zodiac. He was the the one person, outside of law enforcement, who was especially knowledgeable about the case. IMO, this is why he was singled out.

The Halloween card was, in relative terms, a desperate plea to be understood. Of course, the precise meaning of the card is open for debate. However, I think most people would agree that even though we do not know the exact meaning of the card, it is likely an important clue that does have some type of substantive meaning. The Zodiac was hoping that Avery would figure it out – use his context knowledge to unravel the mystery.

Unfortunately, everybody concluded the card was a threat against Avery (the detectives, Avery, etc.). Coworkers made "I am not Paul Avery" buttons. Avery began carrying a firearm (for a time). And the mystery woven into the contents of the Halloween card remained, and remains, frustratingly intact.

This is very relevant thinking and I’m pretty much in agreement. It’s important to remember that this was sent to Paul Avery and therefore there is a high degree of likelihood that whatever the message or messages contained in it are, they were meant for him. That’s what I like to think anyway and convince myself I’m being logical.

However there is something in the my head that also contradicts this. Well I say contradicts but it’s probably more of a paradoxical partnerships of sorts. We know that Zodiac liked the exposure/attention and taking credit for things. He liked to flaunt his so-called cleverness so in that respect I see this as a challenge rather than an attempt to be understood but it could be both, just wrapped up in a way that promotes the former. "You’re so clever, figure this out". Obviously Paul didn’t. Maybe that’s why he didn’t get credit for Riverside in the LA Times letter.

I can’t quite tally Z giving too much of his limelight to someone else either for too long and he probably didn’t but out of all the communications, if you accept most of them then Paul Avery got the most attention. The Halloween card, the pines card and possibly even the LA Times letter. Reading it again in context of the timings (thanks to Wier) he begrudgingly credits the police for the find but he does qualify this as "stumbling across" or rather not of their own doing because they were "sitting on their fat asses" so in a kind of default way he does credit Avery although true to what we imagine of Z’s ego, he can’t bring himself to spell it out. He does however choose Avery to write to again just 9 days later (Pines card) so in that context you could argue that after writing the LA Times letter, Avery was on his mind again.

Not sure about Z knowing anything about Avery’s riverside investigation but you never know. I think given the nature of reporters and their eagerness to ‘get a scoop’ I can’t imagine Paul revealing his hand too much. Also given that the informant understandably would want to ensure their own safety, as the Zodiac was still around, I can’t imagine there being too much information floating around as to what was going on prior to Avery breaking the story. Even at that it might have panned out to be nothing.

Considering Weir’s question about this attention that Avery received and if it was somehow due to something that had angered or annoyed Zodiac, MikeC might have mentioned a possibility, as a matter of course, in his reply. The buttons. Wouldn’t it be so typically petty of someone with Z’s ego to not get annoyed that some reporter was uncovering secrets from his past but rather that sed reporter had succeed where he had failed and actually managed to get some ‘nice buttons’ wandering around town. Who know’s maybe he even saw the ironic humor in it.

Some good thoughts as morf said.

I absolutely agree (regardless of why he singled Avery out) that this was a challenge to him by Z, containing something substantial, should he be able to figure it out. In fact, I believe he spelled that out. For me at least, there is one item that sticks out, that for all intents and purposes has no business there, save, it’s inclusion was significant. On the inside of the envelope Zodiac writes " Sorry No Cipher". Why did he need to write that? An apology that he didn’t have one for us this time? I think not. Is he telling us that he went to the trouble of constructing the card in the way he did and it has no meaning? Highly unlikely..and did he believe that people would believe that? Don’t think so. So what’s the purpose of it?
I think the significance of it is contained, in what he wrote and the way he wrote it. He could afterall, have simply written " Sorry No Cipher", however he doubles up and writes it twice in the form of an X. FWIW, I think this "X" represents the Rune Gebo, meaning "Gift". The Gift being twofold, A challenge to Avery, should he be able to solve it and secondly a Gift by way of hinting what’s at play. A No Cipher, which is a None/Null Cipher by another name.

I think the person who wrote it didn’t know how to do ciphers.

There was an article written in the LA Times on March 6th, which apparently mentions Zodiac. I’m wondering if THAT is what triggered the LA Times letter by him. viewtopic.php?f=94&t=134

IF Zodiac had a thing for Paul, he certainly never mentioned it anywhere else. Only to Paul, in completely different formats.

^^That same subject was in an article in the SF Chronicle on March 3rd…..but, it doesn’t appear it was buried on the back pages.

http://zodiacrevisited.com/bibliography/ (thx zodiacrevisited)

The tricky part of the Avery communication was the stamp. Avery had accused Zodiac of involvement in the Bates murder. The stamp on the Avery letter with a picture of the earth from the moon and "In the beginning…" inscription commemorated the Apollo 8 launch on the morning of December 21, 1968. The Faraday-Jensen murders were committed late on the evening of December 20, 1968. The stamp, which broke the pattern of FDR six cent stamps and Jefferson 1 cent stamps Zodiac used may have been Zodiac telling Avery he was wrong and the first murders were Faraday-Jensen and not Bates.

Kinda tangential, but I’ll toss it in the mix. My "POI" had a degree in Graphic Design. Re the cards mailed by Z: were they from different companies, or the same one? Anyone know?

Different.

I too agree with the idea the ‘sorry no cipher’ in the form of an X was significant, but for totally comical reasons. This was contained on the inside of the envelope. Maybe Zodiac was being ironic towards Paul Avery, sealing the envelope with a kiss, and placing a pumpkin over the genitalia in a loosely comic way, after Paul Avery’s comment about Zodiac being a latent homosexual. The art of ‘blowing’ Paul Avery a kiss was a touch of humorous sarcasm. Forgive the pun.

Except it wasn’t Paul’s comment, Paul just reported about it as he had reported about Zodiac many, many times before.