Who is the “Concerned Citizen”?

On July 31, 1969, the Zodiac killer mailed three letters to three different San Francisco newspapers. Each letter included details about his recent murders, and a third of his 408-character cryptogram. The pieces of the cryptogram were soon published, and in less than a week, Donald and Bettye Harden had already solved the cryptogram. Their solution was published in the San Francisco Chronicle on August 9, 1969. “I’m convinced he has it solved”, said Detective Sergeant John Lynch, who was in charge of the case at the time.

The Hardens’ solution is well-known, but Sergeant Lynch also received a lesser-known solution to the cryptogram, sent in the mail on August 10th by a “concerned citizen”, and obtained last year via the valiant FOIA efforts of morf:

The card was sent one day after the Hardens solution became publicly known via the Chronicle article. With the card came this sheet of paper showing a substitution key to the 408-character cryptogram:

The FBI examined the key and determined it to be “substantially accurate”:

They also apparently found latent fingerprints on the front of the card:

But no one seems to know who sent the key. Was it from a person who read the Hardens’ solution in the Chronicle, and wanted to be helpful? Was it from the Hardens? Or was it from the Zodiac killer himself? Or maybe another clever solver, independently cracking the cipher around the same time by coincidence?

Each possibility seems to have problems:

1) Let’s say some person read the Hardens’ solution, then decided to put together a handy key to show the substitutions used in the cipher. This key is very accurate, but take a look at this part:

Here, the author of the key is saying “the cipher text symbol Q is used to represent plain text letters F, K, L, and M.” There is a simple error here, because there are actually two Q symbols. The symbol is used for the plain text letter F, and the symbol is used for the plain text letter M. OK, so the author of the key combined both symbols. But where do “K” and “L” come from? How did the key author deduce those from the Hardens’ plaintext solution?

2) Let’s say the key was sent by the Hardens. Well, why would they send a card asking to remain anonymous, a day after their names are already published in the newspapers? Also, the “K” and “L” problem still exists.

3) Let’s say the key was sent by the Zodiac killer. Shouldn’t he have been frustrated that someone solved his puzzle so soon? If he didn’t know about the published solution, why could he only wait a week before spilling the beans? Still, the card shows a few Zodiac-like qualities: the misspellings, similar use of the FDR stamp, saying he wouldn’t give his name, and the use of the word “citizen” (Zodiac used the word in the 1974 “Badlands” letter). If it was him, maybe he was just trying to refine the Hardens’ solution.

Since we have handwriting samples for the “concerned citizen”, the Hardens, and the Zodiac, maybe we can compare them for similarities:

(Click to see the full comparison. Warning – might take a while to load because of many little images.)

I see similarities between the “concerned citizen’s” and Harden’s handwriting. But I don’t think it’s conclusive, and I think it’s more likely to be some other person who interpreted Hardens’ published solution or independently solved the cryptogram. The card seems a bit strange, though. And adding to the mystery is the fact that two pages were entirely redacted by the FBI before the documents were released:

Morf received two blank sheets (one | two) bearing only the case file number, 9-49911-1. What was on those pages?

Who do you think the “concerned citizen” is?