Here's a bit of curious info from page 359...
(In reference to another movie Damon was starring in...)
But because it went over schedule, Corbucci decided he'd have to cast another actor as the lead in DJANGO.
Mark protested: "This is a script we worked on together. We wrote it for me to play DJANGO".
"I know", lamented Sergio, "but I have no choice. We have a start date and you're not ready. I'm sorry".
Instead of Mark "they cast a guy with piercing blue eyes whom they discovered working at a gas station", he recalls with chagrin. His name was Franco Nero.
On down a bit it says this...
Although Mark missed starring in it, at least he received a co-writer credit.
It doesn't say whether he used a pseudonym or had his name removed but I don't see Damon's name listed as one of the writers on the credits of the film.