This should make you think, when you consider the April 20th 1970 letter arriving after 121 days sf silence from the Zodiac Killer, mailed on the same day his Zodiac pseudonym was reported in the San Francisco Chronicle as being written in blood on the wall of murder victim Robert Michael Salem, stabbed to death in his 745 Stevenson Street apartment. Was this just sheer coincidence after four months, or was the letter a rushed response to the breaking news in the Chronicle newspaper on the same day (with an AM postmark)? If you play the odds, the chances that this was coincidence, may appear unlikely. If the April 20th 1970 letter was a response to the San Francisco Chronicle article, then the cipher and bus bomb diagram were literally created in a matter of hours. Of course, the 13-Symbol code could have been designed in advance and added to his letter - but if not - the message contained within the code could likely be a reaction to the latest news about Robert Salem, or at the very least, be extremely contemporary in nature if his other three ciphers are anything to go by.
In fact, if you believe the 148 character cipher and letter in 1971 as genuine Zodiac material (as I do), then there will never be any reasonable chance of the Zodiac Killer ever revealing his real name in any code or communication, because the Zodiac Killer comprehensively stated in the 1971 letter that "I will never give my name because you don't understand". The offering of an alternative name in the Z13 seems equally unlikely, because its lack of ciphertext characters lends itself to thousands of possibilities, rendering any alternative name a pointless exercise when it cannot be verified. There appears two ways forward:  either the Zodiac Killer fashioned a code based on the Robert Salem murder or its column inches, so contemporary in nature, that it would carry great significance when unearthed, or  It isn't necessarily contemporary in nature and the answer or key lies in subsequent communications. The Zodiac Killer would almost certainly have known a code of just 13 characters was practically unsolvable, so the notion of a solve so contemporary in nature as to cement its validity, appears a justifiable avenue to explore.