The Zodiac Killer actually proclaimed all four attacks in the form of letters, phone calls and the writing on the car door at Lake Berryessa, yet seemingly continued adding to his total undiminished, to the point of 37+ upon the arrival of the Exorcist Letter on January 29th 1974, but failed to back any of these claims up, other than a loose reference in the Los Angeles Letter of "I do have to give them credit for stumbling across my riverside activity, but they are only finding the easy ones, there are a hell of a lot more down there".
An inexorable leap of 20 victims accompanied by not one letter, from a killer with an insatiable appetite for the dramatic and unrelenting narcissistic qualities, offset by the proclamation of Me - 37+. Is there just a sliver of doubt that all the supposed Zodiac communication subsequent to 1971 or even as far back as from the Little List Letter on July 26th 1970 is simply not from the Zodiac Killer. We have a killer unaware of DNA, proclaiming in the Bus Bomb Letter "As of yet I have left no fingerprints behind me contrary to what the police say. In my killings I wear transparent fingertip guards. All it is is 2 coats of airplane cement coated on my fingertips -- quite unnoticible + very efective.". Whether or not this was the case regarding the fingerprints lifted from the taxicab of Paul Stine, you would not expect him to make the same mistake twice, let alone conveniently leave us a writers palm print, squarely deposited on the front of the Exorcist Letter, once again regurgitating the Mikado reference to imply its authentic qualities.
The Exorcist Letter may well have been authentic Zodiac correspondence, but the leaving of a palm print from such a seasoned letter writer, who would surely be learning and not regressing seems a little unfortunate.
Here is the current evaluation of the Exorcist Letter and below is the link to the polling votes for nine other Zodiac missives.