This was noted in the San Francisco Chronicle on October 12th 1970, just 15 days prior to the mailing of the 'Halloween' card: "It was immediately apparent that Zodiac had plagiarized several stanzas from an aria in the Gilbert and Sullivan operetta 'The Mikado'. It is the entrance aria of Ko-Ko, the Lord High Executioner. A quiet search of onetime Ko-Ko's has turned up none that could be Zodiac. Obvious differences in physical description and handwriting comparisons have cleared all Ko-Ko's tracked down since the arrival of the July 27 letters."
The text on the Exorcist letter is split into three sections:  The introduction referring to the recently released Exorcist movie, and featured in a newspaper article by reporter Paul Avery,  The latest Zodiac pseudonym disguised under the heading of 'Yours truley', and  The customary threat promising more victims if his "note" is not published.
There is little doubt that the Zodiac Killer (now Lord High Executioner) is describing the William Friedkin movie 'The Exorcist' in terms of 'The Mikado'. The Gilbert & Sullivan play is often described as a comedic opera of political satire, so the author choosing the words "I saw and think "The Exorcist" was the best satirical comedy that I have ever seen" is certainly not by accident. However, it is the introduction of "I saw and think" that may carry more meaning than initially thought.
'As Some Day It May Happen' featured heavily in the July 26th 1970 letter. It was spoken by Ko-Ko, the Lord High Executioner, with the word "think" appearing four times during this act, including the introduction :
 "Gentlemen, I'm much touched by this reception. I can only trust that by strict attention to duty I shall ensure a continuance of those favours which it will ever be my study to deserve. If I should ever be called upon to act professionally, I am happy to think that there will be no difficulty in finding plenty of people whose loss will be a distinct gain to society at large".  and  "And who doesn't think she dances, but would rather like to try; And that singular anomaly, the lady novelist — I don't think she'd be missed — I'm sure she'd not be missed".
 "He's got her on the list — he's got her on the list; And I don't think she'll be missed — I'm sure she'll not be missed".
The third section of text on the 'Exorcist' letter is uncannily similar to the first letters mailed by the Zodiac Killer on July 31st 1969. The 'Exorcist' letter was mailed to the San Francisco Chronicle, so we will take a look at the July 31st 1969 letter mailed to the same paper. This is what the killer signed off with: "I want you to print this cipher on the front page of your paper. In this cipher is my idenity. If you do not print this cipher by the afternoon of Fry.1st of Aug 69, I will go on a kill ram-Page Fry. night. I will 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." It begins with a demand to print his material in the paper, and finishes with the threat "to kill" again if his instructions are not followed, exactly like the Exorcist letter.
The 'Exorcist' letter symbols were decoded by Kevin Robert Brooks to spell the words "To Kill", which although not proven, makes perfect sense with respect to the July 31st 1969 letters, and in keeping with the Lord High Executioner's list of people who will not be missed. The Mikado style symbolism at the foot of the Exorcist letter threatening "To Kill" again, lending credence to the notion the Zodiac Killer is now operating under the pseudonym of a Japanese High Executioner.
The answer that has always eluded people - is did the Zodiac Killer begin his murders and letter writing in Riverside, before resuming his campaign of terror in the Bay Area? The Zodiac Killer communications began on July 31st 1969, and presumably ceased with the last confirmed correspondence on January 29th 1974. We have shown the similarity of threats exhibited by the Exorcist letter and the trinity of July 31st 1969 letters, regarding his promise "to kill" if his cipher or "Tit-willow" communications were not published. The 408 cipher was split into three parts (similar to the Exorcist letter) containing 8 lines of text, thereby 24 lines in total. The verse Tit-willow was split identically to the 408 cipher, three verses of 8 lines. The three Bates letters were mailed to Joseph Bates, the Riverside Press Enterprise and Riverside Police, and once again contained double postage like much of the Zodiac correspondence, with each containing just 8 words: "Bates/She had to die there will be more".
The Mikado influence is present in all three sections of the Exorcist letter (including the Asian style characters) in a reinvention of the July 26th 1970 Little List letter. Was the Exorcist letter a hoaxer operating under the guise of the Zodiac Killer, plagiarizing the Gilbert & Sullivan opera once again and convincing us the threat was still alive in the Bay Area after nearly five years, or was the executioner at Lake Berryessa on September 27th 1969 biding his time, before extinguishing the Zodiac Killer once and for all?