His next confirmed letters were the 'Exorcist' letter and the 'SLA' letter, and it may just well be that all these four pieces of correspondence were interlinked, before ultimately sending us the final key, when he mailed a card from Eureka, CA in the December of 1990, hence the name, the 'Eureka' card. One cannot help thinking that this card, devoid of his handwriting, was the Zodiac Killer giving us an unexpected solution to an earlier problem in the form of two photocopied keys, let's say his Eureka moment. But it may just have began with the 'Halloween' card personally addressed to Paul Avery.
On the front of the 'Halloween' card the Zodiac Killer had not only written the number 14 on the skeleton's outstretched hand, but had also written 4-TEEN on the inside of the card. This may well have alluded to a potential 14th victim, but other links would be made when the 'SLA' letter, mailed on 14th February 1974, arrived at the San Francisco Chronicle with connections to Paul Avery once again.
Paul Avery had covered the unfolding story regarding the kidnapping of newspaper heiress Patty Hearst (19), taken hostage on February 4th 1974 by the Symbionese Liberation Army, an urban militant group. She later would become viewed as a sympathizer for the group, after her involvement in a bank heist along with other SLA members. She was caught on the bank's security cameras on April 15th 1974 brandishing a M1 Carbine, and appearing a willing participant in the robbery.
Seen on Wikipedia, it is notable from the first paragraph, that Paul Avery was "best known for his reporting on the infamous serial killer known as the Zodiac, and later for his work on the Patricia Hearst kidnapping". The Zodiac Killer evidently knew this, responding with the SLA Letter ten days after the kidnapping, stating "Did you know that the initials SLAY (Symbionese Liberation Army) spell "sla", an old Norse word meaning "kill". a friend." Additionally, this letter and the 'Halloween' card have been the subject of comparison many times, regarding the mailing date of the 'SLA' letter on Valentine's Day the 14th, lending to the VAL and LAV connection, albeit reversed. However, reversing or flipping text was not unusual for the Zodiac Killer in his later communications, apparent in the October 5th 1970 '13 Hole' postcard and March 22nd 1971 'Pines' card. So we now have definite connections to Paul Avery through the 'Los Angeles' letter, 'Halloween' card and 'SLA' letter, leaving us to attempt a connection of Paul Avery to the 'Eureka' card, mailed in the December of 1990.
The link may prove tenuous, but what we do know about this card, is that it starts with exactly the same line as the 'Halloween' card: "From Your Secret Pal," and therefore could be regarded as a sister card. It did however contain a strange addition, or possibly an important clue, a Xerox or photocopy of two keys on a chain.
Bearing in mind the Zodiac Killer's ability to flip or reverse his text in his latter correspondence, it would not be surprising if the Xerox was suggestive of such an approach in the 'Halloween' card, regarding the LAV in Paul Averly being reversed to give us VAL.
"I have not found it," the solution that is, but I still remain positive about the negative.