Over the years, many people have tried to explain the meaning of the card, the symbol, etc, but it was all open to speculation. First of all, the Zodiac had placed several eyes on the card. A day or two before Zodiac mailed this card, a female was found murdered in the Presidio Heights neighborhood of San Francisco. Her eyelids had been sadistically sliced off. I personally think that Zodiac included the eyelids on the card, and mailed it card immediately after the body was found, so that police would think he had killed her, afterall, it was the same neighborhood in which Zodiac had killed cab Driver Paul Stine, the October before. It was not long before the Police identified the female victim as a prostitute,and her Pimp was found to be responsible for her murder, NOT the Zodiac. Although the eyelids may have been easy enough to explain away, the other clues on the Halloween card, were still of interest.
Regarding the odd symbol, that resembled a combined Z&F, with 4 dots sorrounding the center, Author Robert Graysmith, in his book, ZODIAC UNMASKED, noted that the sybmol was a close match to the cattle brand symbol used by Fred Harmon, a Rancher from Pagosa Springs, Co. (Top right of the photo below)
Fred Harmon was associated with a comic book based on his western experiences. In December, 2013, TAHOE27 discovered that there was possibly yet another connection between Fred Harmon & Zodiac. In one comic,there is a 'wheel of doom'. The DOOM part is interesting of course because of Zodiac using that word in his threat to Avery. But even more striking, is what the wheel of doom looked like in the comic book. It closely matched Zodiac's clues involving knife,rope,gun,fire.
The connection regarding the symbol, and comic book between Zodiac & Fred Harmon, raises more questions than answers, but one thing is hard to deny, there IS a clue there someplace.