Going with the redo theory, would they have re-filmed the lettering or just recomp'd as maybe some new development was figured out in optics?
Looking at crawl production pics:
SW - (early crawl).................................... ESB
The light rigging box looks the same between movies. The cameras don't look the same but since I can't name the parts, not sure how much is just redressing.
poking around, Sam Longoria says:
http://emusingmusic.blogspot.com/2011/05/typographic-rage.html
The artwork for a filmed title sequence, while hand-assembled from Letraset rub-on letters, as you assert, were not silk-screened onto a glass plate.
They would instead be shot on a stat camera, onto high-contrast Kodalith film. That's pretty standard, for movie title work.
The artwork that went before the ILM VistaVision Dykstraflex motion-control camera, performing its title crawl, (operated by Richard Edlund in your photo), was most probably a big black strip, assembled from sheets of Kodalith film (with white titles on a black field) and black paper, all held together by black photographic tape.
That would be taped onto the top surface of a fan-cooled Light Box, mounted atop the Dykstraflex's main rail, on the floor.
I've shot lots of titles on animation stands, and I was on hand at ILM in 1980, as they shot the "Episode IV: A New Hope" crawl, to replace the original "Star Wars" title.
What I saw that day looked to be Kodalith film artwork on the light box, as it shot that artwork onto VistaVision 35mm color negative.
White letters on a black field, to be composited in the optical printer over the starfield. Yellow title color would be achieved at that stage, as you mentioned.
So let's get our thoughts together, and send over a question or two. The blog is still active.
As Richard Edlund's in the first shot, he's got a website too: http://www.richardedlund.com/home