The upper left corner of an envelope is usually reserved for the sender's name and address, but the author of the Channel 9 letter has placed 1234567 All Good Children Go To Heaven (abbreviated) from The Beatles You Never Give Me Your Money lyrics, as well as drawing the Apple logo from their record label in the bottom left corner. However, we are left with the boxed off section in the top left corner of the envelope, which if a mirror image just like the other writing, should be the acronym AIC. Was the answer present in the July 19th 1978 letter, when he twice stated "I AM THE ZODIAC AND I AM IN CONTROL", or are we looking for a continuation of The Beatles theme regarding the letters AIC.
If we consider one author responsible for the April 24th 1978, May 2nd 1978, July 19th 1978 and March 8th 1981 letters - and he bought several "blocks of four stamps" of the Americana Series in one visit - then we may be able to track the selection of his stamps in respect to his mailings. The Americana Series stamps followed a very uniform plan that outlined the designs of individual stamps as well as the blocks of four. In all, there were five different blocks, each with a different theme, but similar design style – the subject in the center, and the title curving around two of the edges. When all four stamps were placed together, the titles created a frame around the stamps. The five themes were Roots of Democracy, Rights and Freedoms of the American People, Symbols of America, Pioneer America, and America’s Light. Mysticstamp.com.
If the Zodiac Killer was to purchase several of the Symbols of America Series (shown below), these would have been the appropriate postage for the time. The postage was 13c from December 31st 1975 until May 29th 1978, when it switched to 15c. Then on March 22nd 1981 the postage increased to 18c. So three of these stamps would be the correct postage for his three 1978 letters and March 8th 1981 letter, had he applied just one to each.
The Zodiac Killer would be left with one redundant 13c Eagle and Shield stamp in respect to correct postage for any letter up to one ounce. If another potential Zodiac letter is discovered to have been mailed between April 24th 1978 (I Am Back With You letter) and May 29th 1978 (when the 13c postage increased), and it contains an Eagle and Shield stamp on the envelope from the Symbols of America Series, we have to consider it as a strong contender for the same responsible.
If the above letters were just mailed by random hoaxers unconnected to one another, it would be rather unusual to expect three independent and consecutive communications claiming to be the Zodiac Killer, to have all contained three Symbols of America stamps from the Americana Series. If the Zodiac Killer kept a secret stash containing his writing paper, marker pens and a supply of stamps for "future use", away from prying eyes, then it is not difficult to understand how the sequence of his mailings could have unfolded.