This threat was extremely similar to the October 13th 1969 letter, in which the Zodiac Killer claimed "School children make nice targets, I think I shall wipe out a school bus some morning. Just shoot out the front tire + then pick off the kiddies as they come bouncing out". At no point in either the October 13th 1969 letter or the Palo Alto telephone call did the perpetrator claim anything other than "picking off the kiddies as they come bouncing out" or "picking the kids off as they get on the school bus". The emphasis was squarely on shooting schoolchildren in close proximity to a school bus.
The mailing of the Bus Bomb letter on November 9th 1969 was the first time the Zodiac Killer suggested he was going to blow up a school bus by use of a bomb. He mocked the police who believed he was going to "pick off kids" by use of a firearm. The Zodiac Killer stated "If you cops think I'm going to take on a bus the way I stated I was, you deserve to have holes in your heads". He continued "What you do not know is whether the death machine is at the sight or whether it is being stored in my basement for future use. I think you do not have the manpower to stop this one by continually searching the road sides looking for this thing. + it wont do to re roat + re schedule the busses because the bomb can be adapted to new conditions". It was painfully clear that the Zodiac Killer was intimating the use of a bomb to kill schoolchildren. However, the November 9th 1969 communication may not have been the Zodiac Killer's first contact with police with respect to blowing up a school bus. This threat was telephoned through to the Santa Rosa Police Department a matter of days after the Paul Stine murder on October 11th 1969. The call arrived on, or shortly before October 15th 1969.
This article was released on Tuesday, October 21st 1969, indicating that the threat Fred Sowash was aware of last Wednesday, must have been delivered by the man claiming to be the Zodiac Killer on October 15th 1969 at the latest. The telephone threat could conceivably have been delivered as early as October 14th 1969, the day before the October 15th 1969 article in the San Francisco Chronicle, which first reported a connection between the Paul Stine murder and the Zodiac Killer. But crucially, the October 15th 1969 Chronicle article withheld the threat of "School children make nice targets, I think I shall wipe out a school bus some morning. Just shoot out the front tire + then pick off the kiddies as they come bouncing out", for fear of stateside panic. This meant that any telephone caller to Santa Rosa Police Department on October 15th 1969 (or slightly earlier) threatening to blow up a school bus and kill schoolchildren, gave this information over the phone before any threat on schoolchildren was ever released in the newspapers. This caller effectively pre-empted the last paragraph of the Stine letter in absence of its contents being published. This almost certainly means, that the telephone caller claiming to be the Zodiac Killer on October 15th 1969, was the Zodiac Killer. He not only threatened schoolchildren before the full Stine letter contents were made public, but he threatened the use of a bomb to attack a school bus, twenty-five days before the Bus Bomb letter was postmarked.