The easiest difference to spot is that the M1911 has a straight backstrap whereas the A1 has a slightly bulged mainspring housing. No difference in function, but the A1 fits the hand a bit better. Two other, not-so-obvious differences are the shape of the hammer and grip safety. Both changes were made to make the gun more shooter-friendly and do not affect function.
There is a major functional difference in that the 1991 version has a firing pin block safety the standard 1911 does not.
Firing pin block 1911s the triggers arent as crisp as the older style as the have to also push the block up and out of the way. Its only noticible if you have a long history of shooting 1911s without.
Its an unneeded set of parts in a properly functioning 1911. A properly maintaned and correctly carried 1911, round in the chamber hammer cocked with safety on, has a halfway stop to prevent hammer strikes if the gun is dropped. A firing pin safety is only required if the gun is being carried incorrectly withthe hammer down on a loaded chamber