Bearing in mind this article was considering a Zodiac connection, did the Bay Area murderer respond by creating this letter and mailing it almost immediately. The killer hadn't written since the 'Melvin Belli' letter on December 20th 1969, exactly 122 days ago. Therefore, do we conclude that  the Zodiac Killer just happened to independently write the April 20th 1970 communication, which accidentally coincided with the April 20th 1970 San Francisco Chronicle article, but he wasn't the murderer of Robert Salem.  He was the murderer of Robert Salem and had prepared the letter from the time of his death on the 15th, before mailing it on the 20th, or  He immediately sprang into action on April 20th 1970 after reading the article, and created the '13 Symbol' cipher in double quick time, designed his 'photoelectric switched' bus bomb diagram, and recalled the murder of San Francisco police officer Brian McDonnell on February 16th 1970, all in time to have this letter postmarked on the same day. To answer this question we have to go back six months.
Michael Cole wrote "Clearly, the Zodiac did not send the cryptogram to Marsh, as had been requested (this non-public way of satisfying the challenge was apparently unacceptable to the attention-craving serial killer). Neither did he respond in a time frame that most would have expected. But these details matter little. In fact, the timing itself provides yet another reason to conclude that what we have here is a specific instance of cause and effect. As mentioned, this article was published in the Examiner on October 22, 1969. Six months to the day later, April 22, 1970, the people of the San Francisco Bay Area were reading about the My Name Is cipher in the Chronicle. This relationship between the Examiner article and the My Name Is cipher may well mean there is an even higher likelihood that the cryptogram does indeed encipher some form of the killer's name"
If the April 20th 1970 letter was referring to the Robert Salem murder, by teasing us with the statement "I hope you have fun trying to figgure out who I killed", then any manufactured correlation between the dates by the Zodiac Killer would rely on the Zodiac Killer committing the murder just shy of the six month anniversary and then mailing the letter. This would mean the Zodiac Killer could prepare the letter in the days between the murder and the mailing date of April 19th or 20th, to enable it to be postmarked on the 20th. If he didn't kill Robert Salem, and was simply responding to the San Francisco Chronicle with an unprepared letter, then his code and bus bomb diagram must have been designed with little forethought, and mailed in time to be franked on the same day. The other possibility, is he had the code and bus bomb diagram already prepared in advance and just added the text around these designs. However, there is one major problem. The photograph of the crime scene wasn't shown until a follow up article by Paul Avery in the San Francisco Chronicle on April 21st 1970. By which time, the Zodiac Killer had already mailed the letter.
The reason behind this claim was the correlation between eight characters on the final row of the 340 cipher and eight characters from the '13 Symbol' cipher (six being the circled eights).
The 340 Cipher was the Zodiac Killer's unsolved masterpiece, and being a man with an inflated ego, driven by self publicity, what better way to drop us a hint by placing his signature or pseudonym on the final line of the 340 Cipher. You will notice he placed the Zodiac crosshair symbol, followed by what could be interpreted as Zodiac and then an upside down Aries symbol, with a line on the top and a dot to the left. So what we shall do is place the correct spelling of Zodiac alongside the version he placed on the 20th line of the 340 Cipher and use the identical technique we used to create the symmetry on the '13 Symbol' cipher. The alphabetical difference of ZOD produces three zero's, the IAC produces 888. These two sets of figures incorporated gives us the three circled 8's found on his next cipher, as though he was carrying them forward. It is also apparent on the '13 Symbol' cipher that either side of the three circled 8's are the Zodiac crosshairs and an inverted Aries symbol, identical to the final line of the 340 Cipher. So, the author has effectively carried eight symbols from one cipher to another.
"In Christian numerology, the number 888 represents Jesus, or sometimes more specifically Christ the Redeemer. This representation may be justified either through gematria, by counting the letter values of the Greek transliteration of Jesus' name, or as an opposing value to 666, the number of the beast." Wikipedia.
The ZOD and IAC are separated. The first produces the 000. The second produces the 888. The two are then combined, to create the transition from the 340 cipher to the '13 Symbol' cipher. This was presumably designed by the Zodiac Killer when he created the 340 cipher sometime before November 8th 1969, with a view to his next cipher.
When the Zodiac Killer mailed the April 20th 1970 letter and code (assuming he wasn't the killer), he hadn't seen the way Zodiac was written on the wall of Robert Salem's apartment. Therefore, did the killer of Robert Salem just accidentally separate the ZOD and IAC of ZODIAC, thereby mimicking the Zodiac Killer's 340 cipher creation, when he had ample room on the wall to write it normally, or was the Zodiac Killer present at the 745 Stevenson Street, San Francisco residence on April 15th 1970, giving him the opportunity to create the design, and at least five days to prepare his letter? The April 20th 1970 code being the confirmation of his involvement in the murder. Was this the reason for the wording "I hope you have fun trying to figgure out who I killed"?