Thanks Dave for the info!
The current EMP museum special attraction is "Star Trek: Exploring New Worlds". The Star Wars exhibit may no longer be on display :( But using search terms for the museum names you mentioned, i did find lots of great pics of the Star Wars exhibit when it was in Seattle and on the road. For a while it looks like the ESB Stunt Vader was replaced in the exhibit with a ROTS Vader. Blechhhh. I included below a nice close-up of the ESB stunt while on display I found online (probably taken from a distance of 2-3 ft).
I've seen the pics the Vadermoney replica stunt ESB helmet which for a while I'd mistaken for the screen-used one. Very accurately done!
Of Paul Allen's extensive archive nice that he avails them for public viewing (for the price of admission). The Stunt Vader and Alien props are amazing. In photos of the full size "Big Chap" Alien Xenomorph I can finally see the human face in the front of the translucent skull not clearly seen in the movie.
Interesting that the ESB Stunt and ROTJ Stunt may have been made at the same time. Cook while the griddle is hot, so to speak. I would guess the ESB Stunt (Half Fat Lip) was made first (which incidentally follows a similar lower lip droop as the DPDLX...hmmm). Then that was fixed when making the ROTJ Stunt to level it out creating the Fat Lip. Shaping the Perspex to meet up with the fiberglass probably wasn't easy!
Yes I believe the ROTJ stunt is in the Lucasfilm archive. In the promo video for Brandon's Star Wars Costumes book he is shown holding the mask.