I think the Walker was the most powerful handgun until the intro of the .357 Magnum, not the .44.
Also, while I had thought some of the Harry's .44s looked suspiciously like .45s when viewed from the muzzle end, I don't see that Smith offered the M25 in .45 Colt until the 125th Anniversary gun (25-3) came out in 1977; way too late for Harry to use in the first film. Even then, that gun had the shorter cylinder of the 25-2. Maybe just a 25-2 barrel, screwed onto a 29 frame?
Also, while I had thought some of the Harry's .44s looked suspiciously like .45s when viewed from the muzzle end, I don't see that Smith offered the M25 in .45 Colt until the 125th Anniversary gun (25-3) came out in 1977; way too late for Harry to use in the first film. Even then, that gun had the shorter cylinder of the 25-2. Maybe just a 25-2 barrel, screwed onto a 29 frame?