Yeah, and Eastwood even made some changes in the dubbing studio (eg. he suggested changing Sentenza's name to Angel Eyes; if you look closely at the actor's lips, you see them saying "Sentenza"; also, you can see Eastwood's lips saying the original line "IDIOTS -- it's for us" before changing it in the dubbing studio to "IDIOTS -- it's for you." (Thanks to Frayling's Blu Ray disc commentary for that).
I think I recall reading about how Age/Scarpelli's stuff wasn't used -- probably a statement that Leone made to Frayling -- but I wonder if that's really true; if so, why would they have been given credit for the screenplay?
In OUATIA, Leone hired Normal Mailer to write the script; Mailer did a significant amount of work but (as I recall reading in STDWD), Leone ultimately did not use it, and said that hardly any of Mailer's work made it into the final movie -- and Mailer's name is nowhere to be found among the 6 (!) screenwriting credits. Well that seems similar to what supposedly happened with Age/Scarpelli with GBU.
Sergio Donati and others who worked for Leone seem to really resent the fact that he was stingy about giving credit to those who worked for him; Leone even refused to give credit to Donati for his work on the GBU screenplay (I guess you'd have to call Donati an "uncredited script doctor"?) So why would Age/Scarpelli get the top screenwriting credit if their material really wasn't used? Was Leone being generous to them? Was it in their contract that they had to get a screenwriting credit? Was Leone trying to cash in on their popularity? Or did they indeed contribute to the final product, despite what Leone later said?
Like so many other things, we may never know....
(Maybe someone can ask Vincenzoni?)
I actually just took a look again at the GBU chapter in STDWD, and it indeed seems that hardly anything of Age-Scarpelli was used, I think Frayling sais that this matter is one thing that Leone and Vincenzoni agree on!
So in that case, I wonder why Leone gave them a screenwriting credit. Was he contractually obliged to do so? Did he hope that would help to sell the movie