Zodiackillersite.com

Regarding the Los Angeles letter, latent fingerprint section is requested to compare the enclosed palm prints of individual with the unknown prints in instant matter. Results should be furnished to Los Angeles and Sacramento. In later files the subject was ruled out as the contributor of the palm prints. If they were comparing palm prints of an individual with regard to the Los Angeles letter, then this may suggest palm prints were deposited on the Los Angeles letter - and were of sufficient detail to rule out individuals. If this were the case (and assuming the Zodiac Killer wrote with the same hand), why can't they compare the palm print on the Exorcist letter to the Los Angeles letter, in order to not only verify the Exorcist letter, but to dispel any notion that David Toschi could have been the author of the January 74 letter.

If there's a print on this letter it makes me suspicious that, like the Exorcist letter, it may be fake.

That is my reading of this document. But prior to finding this FBI file I'd never heard of palm prints on the Los Angeles letter.