The statement above is one that argues against itself, concluding that the author of the 1987 Vallejo Times-Herald Halloween letter would choose to hoax a hoax letter. They declared it a clever copy of a 1978 hoax letter. Therefore, the 1987 copycat, copycatted a copycat of the real Zodiac Killer, when it would have been far simpler for the copycat to choose any number of authenticated Zodiac letters from either Robert Graysmith's 1986 book or any good library archive. Instead, we have to believe the 1987 copycat made the schoolboy error of choosing one of the few unauthenticated Zodiac letters ever released, and widely touted as such in many newspaper articles.
Robert Prouty added "They appear to be composed of previous letters which have since been made public". Does this mean that the 1987 San Francisco Chronicle communication was determined to have been composed of writing mimicking letters other than the 1978 correspondence? The emphasis here, is that it seems handwriting analysts compared the writing on the 1987 letters with previous letters - and not the handwriting on the 1987 Vallejo Times-Herald envelope with the only other envelope ever mailed to the Vallejo Times-Herald, on July 31st 1969. The similarity and composition of both the 1969 and 1987 envelopes may have changed their determination.
It is my understanding that the July 31st 1969 Vallejo Times-Herald envelope was not released into the public domain, so the author of the 1987 Vallejo Times-Herald envelope could not have mimicked it. However, he did a pretty good job, even using the full address of 'Vallejo Times Herald' on both occasions, in contrast to the publicly available 'Valejo Times' in Zodiac's July 31st 1969 letters to the Chronicle and Examiner. If anybody can find the July 31st 1969 Vallejo Times-Herald envelope in any newspaper prior to the October 28th 1987 letter, please let me know in comments. I do not want to draw any unnecessary conclusions based on incorrect understanding. This currently is just a probing exercise.