If the black male was in the vicinity of the payphone at 12:40an, it is not inconceivable that he could have picked up the latter part of the message that the Zodiac Killer was dictating to Nancy Slover when he said "I want to report a double murder. If you will go one mile east on Columbus Parkway to the public park, you will find the kids in a brown car. They were shot with a 9 mm Luger. I also killed those kids last year.... Good-bye". In other words, the black male may have overheard the Zodiac say "They were shot with a 9 mm Luger", which he would later relay to police as "I shot them. I used a 9mm automatic". This version of the phone call was reported in several newspapers alongside the Nancy Slover version. When responding officers at Blue Rock Springs received the information that the payphone at Springs & Tuolumne had been identified as the location of the call, they immediately drove to the payphone to secure it for fingerprint retrieval. Had they scanned the area for potential suspects, it is not inconceivable that they located the black male, questioned him, and received some limited information about the payphone caller and the tail end of his message, which marginally differed from the recollection given by Nancy Slover. This information may have been withheld to protect the eyewitness.
It is inconceivable that the payphone would have been rang at 12:47am with the Zodiac Killer still at the payphone, because he would have been standing at, or within earshot of the payphone for a minimum of 6 minutes. However, he must have heard the payphone ring because this detail wasn't released in the newspapers (unless he knew, or surmised that "ringback" was the method of tracing the call). So if he heard the payphone ring at 12:47am and saw the black male being questioned by police beyond 12:54am, one could argue that he was either in the neighborhood on foot, viewing proceedings from a clandestine location, or he was "peeking through his curtains" from a residence or building close by. A killer making a phone call at 12:40am while still in possession of his vehicle, should have been long gone by 12:47am. At the very least, a murderer not local to the area, should have put a reasonable distance between himself, his vehicle and the payphone by the time either of the above scenarios had unfolded. If there was no black male that morning, one has to question the reasoning behind this convenient eyewitness that Zodiac detailed so well.