The designer of the Exorcist letter was seemingly substituting the signature with a verse from The Mikado's Tit-Willow. In other words, it could suggest his name is present somewhere within the verse. The Zodiac Killer (if the author) chose a verse not in keeping with his two previous Mikado selections, As some day it may happen and A more humane Mikado - both of which had threatening overtones of torture and murder. Whereas, Tit-Willow is about a little tom-tit's last reflective moments before his suicidal plunge into a billowy wave. This may suggest the verse Tit-Willow was chosen for an entirely different reason, particularly in view of the preceding line of "Signed, yours truley" notable on the Dear Boss letter signed by Jack the Ripper and postmarked September 27th 1988, just three years after The Mikado opened at the Savoy Theatre. It should also be noted that William S. Gilbert, responsible for the libretto of The Mikado, released Songs of a Savoyard in 1890 featuring many of the songs from their comic operas. Six songs from The Mikado can be found in Songs of a Savoyard, three of which Zodiac chose - assuming of course, Zodiac was responsible for the January 1974 offering.
There is a strange dichotomy evidenced in the letter, where the author begins by referencing 'The Exorcist' movie (1973) and immediately follows it up by paraphrasing 'Tit-Willow' from 'The Mikado' (1885). 'The Mikado' is often described as the best in British satire, as well as a comedy opera, therefore the word usage of the author describing it as a 'satirical comedy' is unlikely to be accidental. Additionally, the attempted blending of these vastly different productions into one correspondence seems forced, as if chosen for a specific purpose.
The libretto or text was plagiarized from William S. Gilbert. The Exorcist is an American horror film released in 1973, directed by William Friedkin and adapted for screenplay by William Peter Blatty, based on his 1971 book. The movie divided audiences and critics alike, as it explored the subject of demonic possession, in this case, that of a 12-year-old girl played by Linda Denise Blair. This masterpiece of evil still remains one of the most iconic movies in the horror genre to this day. It is fairly evident that the three main protagonists in these productions all have the forename "William". It is also apparent that every verse of Tit-Willow ends with "Oh, willow, titwillow, titwillow!", in which the name "Will" can be observed three times. The shortened version of William is also present in "billowy". Did the Zodiac Killer choose the Tit-Willow verse after "Signed, yours truley" for no other reason than it contained his name embedded in the text? The next line after "Oh, willow, titwillow, titwillow!" is "Now I feel just as sure as I'm sure that my name".
In the April 20th 1970 '13 Symbol' cipher, Zodiac stated "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...." The cipher contained three characters twice (A, N, and M) and one character three times (joined O and 8). Therefore, the name "William" gives us a good starting point with two repeating letters.
As an experiment, I looked at the most popular American surnames that could possibly fit the criteria required by the '13 Symbol' cipher and the already inserted "William". Sixteenth on the list was the name Martin, giving us an estimated name of William Martin for the Zodiac Killer. There are many alternative second names that could be applied, but if "William" was the responsible's forename, we would be looking for a 6-letter surname for the confirmed murderer of five.