So, from a picture, some of these signatures are suspect, even knowing that they are 30 years old? This is one of the very few times that the Sig Room lets me down.
Yes David Prowse signs "is Darth Vader" now, and he has since his entire career became the convention circuit. In 1982 when they wrapped up Jedi, he was just Dave Prowse, and he probably was hoping that he was going to get a lot more work, and not just be the guy in the black suit.
Signatures can vary wildly in 30 years. They also will be much more carefully and artfully done in certain circumstances. All that can be honestly said is that there are slight apparent differences in the signatures shown on the poster and the signatures obtained in the last several years.
There is no cause, and no evidence, to call the guy's poster "suspect" twice.