The FBI files explicitly state that the correspondence was "undated", and sent "anonymously". In the absence of any dated or postmarked envelope, this strongly indicates that the three-page letter was hand delivered by the killer to the offices of the San Francisco Examiner at Third and Market Streets.
The Zodiac Killer painstakingly detailed his first two crimes in the July 31st letters "stating some facts which only I + the police know.", so must have been aghast at the statement of Vallejo Police Chief Jack E Stiltz in the Sunday August 3rd 1969 San Francisco Examiner & Chronicle, wanting more facts to prove the letter writer was the killer. This may explain his eagerness in rushing off his three-page letter to the San Francisco Examiner. He may have considered posting it in a public mailbox, but his apparent desire to back up his claims in the July 31st letters and have it published in the Examiner by Monday afternoon, forced him to opt for the direct route of hand-delivery.
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. Wikipedia.
Below, the Vallejo News Chronicle details the Zodiac Killer's fourth letter to the San Francisco Examiner on Monday August 4th 1969,
Why on earth would the sender feel the need to write "no address" at the foot of the letter? Probably because there was no envelope containing the address of the San Francisco Examiner.
If the Zodiac Killer had placed all three pages together, placing the 3rd page facing outwards on the top or bottom, then by folding the letters in the middle twice, the face of his communication would measure approximately 2.5 inches in height. The highlighted pink section would be the visible part of his communication, and where the address of the newspaper should have been written. But because he delivered it directly to the San Francisco offices, there was no need for an address - which is exactly what he wrote.
Does the hand-delivery of this letter tell us anything else about the Zodiac Killer, other than his eagerness for publication? The following is speculative and must be regarded as such.
The Zodiac letters have been heavily dissected to discover the likely home residence of the Bay Area murderer, and so we shall attempt to make some observations regarding this communication.
How prepared would the Zodiac Killer be, to travel a long distance just to hand-deliver this letter, rather than mail it?
If the Zodiac Killer lived or worked in San Francisco, then it isn't a problem to deliver it in person, either by making the journey specifically or en route to his workplace. However, if he lived in an outlying area (but didn't work in San Francisco) such as Napa (44 miles), Sacramento (94 miles), Santa Rosa (57 miles), Vallejo (29 miles), Oakland (9 miles), then how likely would it be for a person to travel these distances for no other purpose than the desire to hand-deliver a letter? Surely, based on the 'least effort principle' of geographical profiling, the further away he lives, the less likely this would be the case
This may suggest that the killer either lived and worked in San Francisco, or lived in one of these other regions and traveled into San Francisco for his profession. If he lived in Napa, for example, would he travel 44 miles (88 miles round trip) for the sole purpose of dropping off a letter, rather than mail it in Napa or somewhere close by? Unless, he lived in Napa and worked in San Francisco.
If the August 4th 1969 'Debut of Zodiac' letter was personally delivered by the murderer to the San Francisco Examiner, does it in some small way reveal something about Zodiac Killer and his likely residence, or is "no address" the likely outcome.