This is nothing new to a lot of people, but I wanted to post this pic and quote from Wikipedia.
The M1911A1 changes to the original design consisted of a shorter trigger, cutouts in the frame behind the trigger, an arched mainspring housing, a longer grip safety spur (to prevent hammer bite), a wider front sight, a shortened hammer spur, and simplified grip checkering (eliminating the "Double Diamond" reliefs). These changes were subtle and largely intended to make the pistol easier to shoot for those with smaller hands. No significant internal changes were made, and parts remained interchangeable between the M1911 and the M1911A1. Since I worked on so many WWII manufactured M1911A1s in the army, the earlier version in this picture looks kind of odd to me. It still has the old-style thumb safety, and although the hammer spur is shorter, it's still wide at the back, instead the same width all the way across. I almost never saw those. The slide stop is the old checkered version too, instead of the cheaper to produce serrated one. And the big circles around the grip screws look really weird. I can't tell if there are washers under the screw heads, or if that's just the way the grips were made. It looks like flat steel washers to me, which I don't recall ever seeing.
By the way, one time I saw an M1911 in a museum listed as an M1911A1, or it could have been the other way around, I can't remember which it was now. I find it funny that so many people who buy or build 1911 style pistols use original style flat mainspring housings, instead of the improved arched mainspring housings. If you put a long trigger in an M1911A1 I'm happy with it. I like arched mainspring housings on single-stack guns and think they point better, but prefer flat ones on my double-stacks because they have enough girth already.
That's what she said.
https://en.wikipedia.org/wiki/M1911_pistol