They learned a few things, during the war.
They found that, with point shooting, the 1911 shot a little low for most people. They changed the mainspring housing from flat to arched, which made the gun point higher when you held it.
They found that some folks had trouble reaching the trigger. They changed the length of the trigger, making it shorter, and they machined little notches into the frame, on both sides, behind the trigger, so short fingers could get in easier.
They found that the hammer spur would often pinch a layer of flesh between the back of the spur and the top of the grip safety, so they shortened the spur slightly and lengthened the grip safety, slightly.
The biggest VISIBLE difference is the notches behind the trigger guard. Everything else is replaceable. You can put a 1911 trigger on an A1, and vice-versa. Same with flat and arched mainspring housings, hammers and grip safeties.