I'm sick all over again
When I left Florida to come to Alaska in 1980, I was offered a pistol and holster almost exactly like yours. In trade for a camera lens which, as it happens, I have never taken a picture through since, but I kept because I was going to drive FL to AK and thought I should keep. Dollar value, about $250. Kicking myself ever since.
"My" .45, too, was from the second production run of 1911s sold to the U.S. It, too, had the flat mainspring housing, long trigger and short grip safety, but mine had been Parkerized (I believe the original finish in the early years was blued.) It was not uncommon for guns after the First World War to have been returned to armories, sometimes in parts, to be reassembled from the parts bins for reissue later (after refiniahing), leading to some 1911-1914 frames being mated with 1945 slides being used in the Korean War in the '50s.
I am still sick over not grabbing that gun, but I knew I was going to have to drive through Canada, so shipping from a Montana FFL to and Alaskan FFL would have been necessary. But I wish I had done it.
I once had a really nice leather bicycle seat that had been left out in the weather for a year. It was curled, stiff and looked a lot like your holster. I worked it with saddle soap every day for a week until it regained its shape, then oiled and waxed it (forget what I used) until it regained its proper feel. It never did look as pretty as it probably did when new, but it rode nicely.
Before I worked on a valuable holster, I would check with a holster maker or saddler or other leather expert for advice.
Good luck. You have a bit of American History and Family History there. I envy you.