Many have asked the identity of the "concerned citizen." Was it Donald Harden, the Zodiac Killer or a third party who broke the code independently? The latter having not been aware of the code being broken, as published in the newspaper.
The Chronicle, according to the August 9th 1969 newspaper article had "already done" sending Sergeant John Lynch the worksheets, so why would Donald Harden be mailing the cipher key through the mail, when clearly he was already in contact with police. Therefore, this option can be summarily dismissed as the least likely option.
The Bay Area murderer, as of August 4th 1969, had already given us his pseudonym with the dramatic introduction of "This is the Zodiac Speaking", so why would he now be acting coy in giving us the cipher key under the guise of a "concerned citizen". This tactic may have surfaced during the 1974 communications, but to implement it so early, just after threatening to "cruse around all weekend killing lone people in the night then move on to kill again, until I end up with a dozen people over the weekend", seemed an immediate backpedal. From "ruthless killer" to "concerned citizen" in a matter of days.
On the other hand, this may have been the start of his game with both the newspapers and law enforcement alike.
The card reads:
Dear Sergeant Lynch.
I hope the enclosed "key" will prove to be beneficial to you in connection with the cipher letter writer.
Working puzzles criptograms and word puzzles is one of my pleasures. Please forgive the absence of my signature or name as I do not wish to have my name in the papers and it could be mentiond by a slip of the tongue.
With best wishes.
Although it has been stated on many forums that the author of the "concerned citizen" card may have mailed it, unaware of the code being broken and published in the San Francisco Chronicle on August 9th 1969, this seems highly unlikely. The author used the wording "cipher letter writer", exactly as published in the newspaper. This, from a grammatical standpoint, is highly likely to have been copied from the newspaper, indicating that the person who mailed in the cipher key already knew it had been broken. This could indicate it was Zodiac after all.
THE CONCERNED CITIZEN CARD [PT2] THE CONCERNED CITIZEN CARD [PT3]
Vallejo Times-Herald - Cryptogram Deciphered by Teacher August 9th 1969