I took a look at this communication yesterday - and not totally convinced that the choice of the card necessarily has to have a meaning - I looked at it in respect to Zodiac's previous letter on April 20th 1970, which was mailed the same day the Robert Salem murder broke in the newspapers (with Zodiac having been scrawled in blood at the murder scene), Impeccable timing by a killer who hadn't written for four months, and stated "I hope you have fun trying to fiygure out who I killed". He didn't kill Robert Salem, but the uncanny timing of this letter, suggested that he wanted police to think he did.
Don Quixote and Sancho Panza have been long been argued for the depiction on the "Dragon card", along with gold prospectors, suggested by Luigi Warren. If anybody can categorically nail down the card designer's intent behind the image, then we may have a greater possibility of determining if any meaning lay behind its selection. Without this, the card is open to many interpretations, just like the Halloween card. Even with the designer's input on the meaning of the image, I have little doubt hundreds of forged connections could be made, depending on what Luigi or I (or anybody else) wanted to sell to the reader.
The story depicts an epic battle: "With the sharp-edged swords of the two valiant and enraged combatants held and raised on high, they seemed to threaten heaven, earth, and the abyss: such was their boldness and bearing. The first to strike a blow was the choleric Basque, and he delivered it with so much force and fury that if his sword had not turned on its way down, that single blow would have been enough to end this fierce combat and all the adventures of our knight; but good fortune, which had greater things in store for Don Quixote, twisted the sword of his adversary, so that although it struck his left shoulder, it did no more than tear through the armor along that side, taking with it as it passed a good part of his helmet and half an ear, both of which, in fearful ruin, fell to the ground, leaving him in a very sad state". Luckily, Don Quixote survived, having a part of his left ear cut off. Robert Salem had his left ear cut off and removed from the crime scene. However, this is a connection, among many, that can be easily fashioned from the "Dragon card". There may be a meaning behind Zodiac's "Dragon card", but I highly doubt this is it. The "Dragon card" has too many variables, open to interpretation. Give me a meaning behind the mythical "Dragon" (possibly from a newspaper article in the days or weeks before April 28th 1970 concerning Zodiac) and we may be in business.
Please visit Luigi Warren's Substack to read his analysis of the Zodiac case.