In this analysis we will attempt to bind the May 6th 1986 and October 28th 1987 letters to one author. That way, if we can prove the validity of one correspondence as being authored by Zodiac (such as the 1987 letter), we automatically validate the other. In the 'Los Angeles' letter from March 13th 1971, the Zodiac wrote "This is the Zodiac speaking. Like I have allways said, I am crack proof. If the Blue Meannies are evere going to catch me, they had best get off their fat asses + do something. Because the longer they fiddle + fart around, the more slaves I will collect for my after life". In the 1986 letter, the Zodiac wrote "This is the Zodiac speaking. I am still out here an crack proof. I want you to know about my latest slaves that I have collected about two weeks ago up by Sacramento Ca".
The author of the 1986 letter converted "I am crack proof" to "I am still out here an crack proof". In other words, he just plagiarized the words from the 1971 letter, which doesn't prove he is the Zodiac Killer. Many letters and cards have been mailed to the newspapers between March 13th 1971 and May 6th 1986. I have noted at least thirteen communications linked to the Zodiac Killer between these dates, and not one used the term "crack proof". The first time this term was used, was in the October 5th 1970 pasted postcard, but it was written as "crackproof" without the separation. That means the author of the 1986 communication selected the words "crack proof" for the first time since March 13th 1971 (15 years later). The author also used a 'Flag Over Capitol' US stamp.
You will also notice that the author of both communications managed to prominently black out two words on each occasion. All four words prominently blacked out over both letters, almost certainly began with the letter T. What is certain, is that the prominently blacked out words were all followed by a word beginning with T. Those four words were They, The, Tell and Toschi. Three individual letters were blacked out over both communications also.
Additionally, the Los Angeles letter of March 13th 1971 stated "If the Blue Meannies are evere going to catch me, they had best get off their fat asses + do something". The 1986 letter author would mimic this narrative, by stating "The Blue Meannies almost caught me". But if the 1987 letter writer was not the author of the 1986 letter, he was again extremely fortunate to use wording similar to that contained in the 1986 letter: "Tell the blue pigs if want me I will be out driving around on Halloween in my death machine looking for some kiddies to run over. The pigs can catch me if they find me out there".
I think it is therefore reasonable to assert, that the 1986 and 1987 letters were written by the same hand, whether Zodiac or not.