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 generated a 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?
From August 4th 1969 to March 13th 1971 the Zodiac Killer mailed 12 authenticated letters (inc. Fairfield letters} that carried the introduction of "This is the Zodiac speaking" on the first line of the message. Of these 12 letters, only the Melvin Belli letter on December 20th 1969 failed to keep the "This is the Zodiac speaking" introduction exclusively on the main first line (ignoring the "Dear" intro). It was written "This is the Zodiac speaking I". The San Francisco Chronicle published at least four of these introductions, which included the October 13th 1969, November 8th 1969, April 20th 1970 and March 13th 1971 letters (shown here).
The author of the April 24th 1978 "I am back with you" letter (if a hoaxer) had every opportunity to just mimic any one of these published introductions, yet he chose to mimic the Melvin Belli letter on December 20th 1969, which broke with tradition. The Melvin Belli message began with "This is the Zodiac speaking I", but was grammatically incorrect in failing to place a comma or full-stop between "speaking" and "I". Therefore, the author of the 1978 letter (if a hoaxer) managed to imitate the message on the opening line of the Melvin Belli letter (including the punctuation error) despite the fact an image of the Melvin Belli letter was not published in any newspapers. Sadly, this is another avenue that will be ignored.
Relevant articles: We Already Have Zodiac DNA, Still Driving Around in 1987 [PT2].
Thanks to Druzer for his great work on this topic of indented writing.
The letter and envelope of the 1978 letter was examined by law enforcement. Here is what was written in the San Francisco Chronicle on April 28th 1978: "Toschi said yesterday that the common, while envelope had no outstanding marks and that the single piece of plain white stationary on which the note was written contained nothing that would give us any indication where he might have gotten it. It was also learned that although the envelope bore a San Francisco postmark, other notations on the cancelation indicated that the letter might have been mailed in San Mateo county or Santa Clara county and was brought here for processing". Law enforcement, at the time, considered this the first contact from the Zodiac Killer since he last wrote the Exorcist letter in January, 1974. If this analysis by law enforcement is correct, then the individual who mailed the 1978 letter, mailed it from the same location as the Exorcist letter, separated by just over four years. They say old habits die hard. Proving the authenticity of the Exorcist letter.