The Zodiac specifically singled out the Exorcist Film in his January 29th 1974 letter, calling it "the best saterical comidy I have ever seen", and somehow combined it with the Mikado, using a verse from Act II-On a Tree by a River. It seems a strange association on the face of it, but it may have been chosen specifically, if you are prepared to suspend reality for just a few short minutes. The first thing one notices, is that the Zodiac appears to have compartmentalized the letter into four parts, followed by a series of strange symbols. In the second section he writes "Signed, yours truley", followed by the Mikado verse. Now this is strange because you would expect a name to follow 'yours truly' at the foot of a letter, so where is the name in this Mikado verse.
So what have the Mikado and the Exorcist movie got in common. Well, funny enough, three things, The Exorcist movie was directed by William Friedkin and adapted for screenplay by William Peter Blatty, based on his 1971 book. The Mikado was the work of Gilbert and Sullivan, or to be precise William Schwenck Gilbert and Arthur Seymour Sullivan. Three William's at the foot of the verse, tying the Mikado and The Exorcist movie into one neat bundle. But surely there has to be more.
Earlier it was pointed out that the Zodiac sectioned the letter into four parts, so we will again concentrate on the verse in the third section, and take a look at the third section of the full verse, shown here on Wikipedia. Below is the third and final verse itself, with an important section highlighted.
Isn't Willow, titwillow, titwillow,
That 'twas blighted affection that made him exclaim
"Oh, willow, titwillow, titwillow!"
And if you remain callous and obdurate, I
Shall perish as he did, and you will know why,
Though I probably shall not exclaim as I die,
"Oh, willow, titwillow, titwillow!"
The Exorcist book was released in 1971 and the movie two years later, so the William Blatty element in the April 20th 1970 cipher had to have another influence. In 1966 William Blatty authored the book 'Twinkle Twinkle "Killer" Kane!' exploring the theme of madness, of housed servicemen , set in the backdrop of the Vietnam War and exploring the notion of whether God exists or not.
In the cipher the first four letters can be rearranged to spell 'Kane' and the final three letters spell the abbreviation for Vietnam, but as I have stated many times before, if you look hard enough, for long enough, patterns will emerge. The only major suspect in the Zodiac case, these findings trigger, is Lawrence Kane, written about extensively by Alex Lewis on his site here. Lawrence Kane served in the US Navy and had documented mental health issues.
The idea behind this article was not to claim this as the answer, but to explore the idea that Zodiac chose the Mikado for either personal or professional reasons, dropping us a hint to his identity along the way, and that brings us on to the 'Little List' letter mailed on July 26th 1970, the only other time he refers to the Mikado. In this correspondence he paraphrases Gilbert and Sullivan's Mikado Act One Part 5a As some day it may happen, performed by Ko-Ko. It refers to the targeting of victims, something the Zodiac knew only too well, however Zodiac also pulls lines from the Mikado, ( A more humane Mikado ), where the author uses the words billiard along with crooked cues and twisted shoes. In this verse you will notice that the Mikado himself is referenced, and one particular line refers to his intentions "To let the punishment fit the crime."
In 1967 DC Comics created The Mikado, a villainous vigilante in the comic book superhero series The Question, by Denny O'Neil and Denys Cowan. He dons a Japanese mask and kills malefactors in appropriate ways – stating "My object all sublime ... I shall achieve in time - to let the punishment fit the crime." The Japanese influence is again observed in the Exorcist Letter symbolism, and we know all about the wearing of disguises at Lake Berryessa.