Take a look at the the four consecutive known letters from April 24th 1978, May 2nd 1978, May 6th 1986 and October 28th 1987, shown below. Now imagine these are from different authors hoaxing Zodiac letters. All four managed to use "indented writing" on the opening introduction of "This is the Zodiac speaking", which was never used prior to 1978 in any of the four published introductions in the San Francisco Chronicle (or in any Zodiac communications). This means that despite having the "This is the Zodiac speaking" introduction available for them to mimic, all four (or multiple) authors failed to keep the introduction in line with the text beneath it (signifying a consistent change in pattern). Subsequent to the introduction, all four authors also began with the personal pronouns of "I" and "You". In three of these the text begins with "I". All of these three examples fail to use correct punctuation between "speaking" and "I". Ideally, these two words should be separated with a comma or full stop (period), but in every instance the author fails to do so, just like the authenticated Melvin Belli letter from December 20th 1969, an image of which was not available to the author of the 1978 letter (if they were a hoaxer). This should prove beyond any reasonable doubt we are dealing with one author who penned the December 20th 1969, April 24th 1978, May 6th 1986 and October 28th 1987 letters. It should also prove it was the Zodiac Killer who wrote at least four of the letters, one of which we know has a full DNA profile. If the 1978 envelope has DNA beneath the stamp and/or envelope seal, why not the others? The 1978, 1986 and 1987 letters are almost certainly genuine Zodiac communications, so why were law enforcement attempting to secure Zodiac DNA from two of the July 31st 1969 envelopes nearly four years ago, when they already have it? Sadly, this is another avenue that will be ignored.
Relevant article: Still Driving Around in 1987 [PT2].
Thanks to Druzer for his great work on this topic of indented writing.