Regarding the tape - twice he mentioned his name in accompaniment. The author stated "I have my real name on a small metallic tape" and "I have my name on the bottom of the lid with the scotch tape". If we are to discover what the Zodiac Killer (if the author) meant by "small metallic tape" we need to travel back to a murder where his name featured, at or close to the attack site, and where it was deliberately left. Fortunately we don't have to go back too far.
On Sunday November 2nd 1975, a man walked up to a couple's vehicle on Monte Cresta Drive, Belmont, California and shot 17-year-old Carlmont High School student Anthony Vincent Bruno Jr to death. "According to Police Captain George Stephenson, Bruno and his girlfriend (16 years) had been parked on Monte Cresta Drive, above Barclay Way in an isolated area of western hills when a man opened the driver's side door and apparently without saying anything fired the shotgun. The girl, who was not identified by police, pushed Bruno's body aside and drove the car to her home from where police were notified. She described the assailant as white, 5 feet 5 inches tall, 140 pounds and wearing an Army green parka with hood, according to police. Stephenson said also that a late model brown pick-up truck, possibly a Dodge or Chevrolet, had been seen in the area at about the time of the shooting." San Mateo Times.
The description doesn't sound much like the Zodiac Killer, but often he would lay claim to murders he never committed. This may have been the case here, when on the following day (possibly having read or heard about the murder) placed something interesting in a phone booth. Mike Morford stated, in reference to a San Mateo news article, dated November 14th 1975 "The reporter, a man named Bob Foster, mentions that on the night of November 3, 1975, the 11 pm TV news on channel KGO, ran a story about a Zodiac letter being found in a phone booth. This is pretty interesting to me for a couple reasons regarding the timing. First of all because, the night before, on 11/2/75, a young man and his girlfriend were attacked on a lovers lane by a gun wielding man. The male victim, Vincent Bruno was killed, but his girlfriend survived. If I recall correctly, there may have been a letter & cassette tape found the same night in a phone booth not far from the attack site, from someone claiming to be Zodiac. Also, I will try to find it again, but in one of the FBI files, there IS mention of a letter & casette tape found together". Here is a previous article covering this topic.
The author of the July 19th 1978 letter may have been referring to this (small metallic) tape, left in the phone booth on November 3rd 1975 alongside the letter claiming to be Zodiac. The author stated "I want you to know it belongs to me and you think I may have left it accidentally". Obviously the tape couldn't have held an audio message from Zodiac, because the author of the 1978 letter wouldn't have stated "I want you to know it belongs to me". But he wanted the San Francisco Chronicle to know it had been left there deliberately. So what could have been recorded on the tape, in keeping with the games of the Zodiac Killer - that also contained his name on the small metallic tape? Maybe something like this?