If you were a hoaxer in 1987, attempting to pass yourself off as the Zodiac Killer, would you really craft a letter in 1987 that virtually mimicked the first three lines of the 1978 letter and expect anything other than a copycat label. The bottom of each correspondence was equally similar, with "yours truly:" and "guess" featuring in both letters. Anybody making a serious attempt at fooling the police and newspapers would surely not have made it so obvious. The author of the 1987 letter clearly didn't care that people viewing this letter would draw the obvious conclusions - and the only person who didn't need to convince you that he was the Zodiac Killer, was the Zodiac Killer himself. These conclusions were made by documents examiner, Robert Prouty, in absence of the comparison between the October 28th 1987 envelope and the July 31st 1969 envelope, both mailed to the Vallejo Times-Herald.
The author of the 1987 envelope, if a hoaxer, would have had no knowledge of the design of the 1969 envelope, because it had never been released publicly by October 28th 1987. So maybe Robert Prouty, Captain Roy Conway and Melinda Stehr should have explained how the author of the 1987 envelope used the same 13 words as the 1969 envelope, used no punctuation in either address, used the full newspaper title of Vallejo Times Herold in both (abbreviated in the published July 31st letters), probably misspelled "Herald" to "Herold" on both occasions (as spelled in the 1990 Celebrity Cypher postcard), and finally managed to create handwriting not too dissimilar (see below). A hoaxer on this occasion would have had to mimic another communication without ever seeing it. Unless of course, it was the Zodiac Killer on both occasions.
Robert Prouty stated "My very firm opinion is that they (1987 letters) were not written by the authentic Zodiac. They appear to be composed of previous letters which have since been made public". Had he stated "My very firm opinion is that they (the 1987 envelopes} were not written by the authentic Zodiac. They appear to be composed of previous letters (envelopes) which have since been made public", he would have been wrong. Had the 1969 Vallejo Times-Herald envelope been released into the public domain by 1987, you can be sure that Robert Prouty would have said the author of the 1987 envelope copied the 1969 envelope. These arguments have been widely used to discredit the 1978 letter and envelope (the latter of which mimicked the Dripping Pen card envelope). If the Zodiac Killer can mimic the 1978 letter in the 1987 letter and mimic the 1969 envelope in the 1987 envelope (which a hoaxer had no access to), then he can mimic the Dripping Pen card envelope in the 1978 envelope, showing that mimicry cannot be used to justify calling somebody a hoaxer.