Keep in mind that the rumor (and it IS a rumor... so take it as you will) was that Fett, even when he was paired down to a single character, had a much more significant and active role in ESB. As much as I would have liked to see more action, it might have destroyed his mystique and I am glad he was left obscurea nd shrouded in mystery. The assumption is that these helmets were indeed leftovers from ESB stunt scenes that never made it beyond the script...
In the Ralph McQuarie panel the speaker who is the writer of the new book they were talking about said that Ralph was given a version of the script where Fett became a main character and that is where the second revisions of the more western looking character came about. The sketch where Fett has a poncho, came from that script.