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 sometime later, 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 a earlier problem, in the form of two photocopied keys, lets say his Eureka moment. But it may just have started with the Halloween Card sent to Paul Avery.
Paul Avery was heavily involved in the reporting of the kidnap of Patty Hearst (19), a newspaper heiress, taken hostage on February 4th 1974 by the Symbionese Liberation Army, an urban militant group. She later was to be viewed as a sympathizer for the group, after her involvement in a bank heist along with other SLA members. She was caught on film, on the bank's cameras on April 15th 1974 brandishing a M1 Carbine, 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 has been the subject of comparison many times regarding the mailing date, not only being the 14th, but Valentines Day, 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, attested by the 13 Hole Postcard and Pines Card. So we now have definite connections to Paul Avery through the Los Angeles Letter, Halloween Card and SLA Letter, so all that is left to do is connect 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 can be regarded as a sister card, but it held an important clue, a Xerox or photocopy of two keys on a chain.
Bearing in mind the Zodiac'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 regarding the Halloween Card, such as the lav in Paul Averly being reversed to give us val once more, or maybe not. The fact of the matter, is that 'I have not found it', the key to the solution remains elusive, but I still remain positive about the negative.