If you check his spelling of the word "victim" as "victom" – following the suggestion he viewed himself as a victim of society before resorting into his murderous acts – and the fact that the last (IMHO) letter from him in 1974 was citing the song "On a Tree by a River a Little Tom Tit" from the "Mikado" (where the "titwillow" comes from) it seems clear to me as his first name was "tom" and he was hinting towards that while his last name probably was beginning with a "z" from where he chose the nickname "z" that will later become "zodiac". It also fits the 13 characters "my name is" cipher from April 20th, 1970 where his name is only the first 3 letters ("AEN") and the "gun target" symbol is actually a "z" that is the first letter of his surname.

Edit: I’ve revisited my theory and now I’m certain it’s "tim": http://www.zodiackillersite.com/viewtopic.php?f=96&t=4009