I had a couple of ideas.

“Curators at the National Film Registry picked the 1977 version of Star Wars to preserve for history’s sake, but they still don’t have a copy in the registry. When they asked for a copy, Lucas refused, saying that he would no longer authorize the release of the original version. The Library of Congress does have a 35mm print of Star Wars, one that was filed in 1978 as part of the movie’s copyright deposit, but the registry, where films are meant to be preserved for history, is still without one.” https://www.theatlantic.com/technology/archive/2014/08/the-star-wars-george-lucas-doesnt-want-you-to-see/379184/

Since George won’t release an original 1977, and Disney hasn’t yet either, this 1978 print seems to be about closest we can currently get to an authoritative copy. I may write to the LOC and ask them what the aspect ratio is. Why not? I won’t cross my fingers for a response, but at least I can try.

The other idea I had was to search for copies of trade magazines from that era. Maybe some of those magazines published a “technical readout” of the films? I will get my library card renewed and see! Keep you posted.