Three letters, detailing his crimes and awash with cryptograms, announcing his arrival to the newspapers, seemed like the perfect opportunity to announce those infamous words of "This is the Zodiac speaking" - yet nothing. His next communication was only sent in response to Vallejo Police Chief Jack E. Stiltz, who doubted the letter writer and killer were the same person and "urged the writer yesterday to send more letters with more facts to prove his connection with the crimes". The Bay Area murderer immediately replied on August 4th 1969, stating "This is the Zodiac speaking. In answer to your asking for more details about the good times I have had in Vallejo, I shall be very happy to supply even more material". It can be seen that this letter would never have been sent, had Vallejo Police Chief Jack E. Stiltz not urged the killer to do so. If the letter had not been sent, then our killer would not have presented himself as Zodiac on August 4th 1969. In fact, he may not have adopted the pseudonym at all.
In absence of this letter, the next letter to contain his pseudonym was mailed two days after the Paul Stine murder on October 13th 1969. So, it's entirely plausible that the muderer of five could have begun and ended his attacks in the Bay Area as a nameless killer. The Vallejo News Chronicle published one-third of his 408 Cipher on August 1st 1969, the San Francisco Chronicle did the same on August 2nd 1969, with the San Francisco Examiner waiting until August 3rd 1969 and publishing all three parts, but on a lowly page nine. All three carried the message from Vallejo Police Chief Jack E. Stiltz, yet despite the Examiner publishing their part of the cipher the latest and relegating it further from the front page than the other publications, in defiance of his demands for front page coverage by Friday afternoon, it was the Examiner he chose to introduce his pseudonym to. In other words, they complied the least, but the killer chose to "supply more details" to them rather than a Vallejo publication and to the home city of Vallejo Police Chief Jack E. Stiltz, who he was supposedly addressing.
There is a real possibility that the murderer of three chose the San Francisco Examiner because they were the only publication to give the murderer a pseudonym, which may have ultimately triggered the following communication a day later. The Examiner wrote "The police telephone system was clogged by anxious callers asking if the "cipher killer" had been caught". The murderer seemingly didn't appreciate the pseudonym given to him and a day later, on August 4th 1969, he introduced himself to the Bay Area with "This is the Zodiac speaking". But why this phrase? The Examiner article noted that "the police telephone system was clogged by anxious callers asking if the "cipher killer" had been caught", so what better way to announce yourself as you might do over the telephone. This wasn't an anxious caller, this was the Zodiac speaking.
The counter-argument to the above, is the killer had ample time to give himself a pseudonym from December 20th 1968 to July 31st 1969, and only began the trinity of letters with "murderer" and "killer" because he had reserved the pseudonym "Zodiac" for the 18 characters at the foot of the Chronicle cipher. This was the portion he claimed held his "identity". A hidden pseudonym would be negated had he began the correspondence with "This is the Zodiac speaking". With the entirety of his communications beginning with this phrase, does it lend credence to the notion its absence from the introduction to these three letters, is validation to its concealment in the 18 unsolved characters? If he hadn't preselected his pseudonym in advance of the July 31st 1969 letters, one could revert back to the notion it was the "cipher killer" attribution that triggered his reply on August 4th 1969. For 35 years starting in 1965, the San Francisco Chronicle and Examiner operated under a Joint Operating Agreement whereby the Chronicle published a morning paper and the Examiner published in the afternoon. The Examiner published the Sunday paper's news sections and glossy magazine, and the Chronicle contributed the features. Circulation was approximately 100,000 on weekdays and 500,000 on Sundays. By 1995, discussion was already brewing in print media about the possible shuttering of the Examiner due to low circulation and an extremely disadvantageous revenue sharing agreement for the Chronicle. Wikipedia. The Bay Area murderer's reply on August 4th, including his pseudonym for the first time, would therefore have been formulated in the mind of the killer from the moment he read the Sunday Examiner publication to his creation of the letter. The Zodiac Killer being born on August 3rd 1969, or early Monday morning on the fourth.
The 18 unsolved characters (if they contain any meaning whatsoever) clearly operate under a different technique of decryption to the other 390 characters. So if the Zodiac Killer didn't happen upon his pseudonym on August 3rd or 4th, then we have the very real possibility that the identity of "Zodiac" does reside in these final 18 characters, to which he alluded to in his address to Vallejo Police Chief Jack E. Stiltz on August 4th 1969. The Zodiac Killer stated "when they do crack it, they will have me".