Hickok45 goes over the difference between the two.
There are several different points, but the most prominent four are
1. Main spring housing is arched in 1911A1
2. area just behind trigger is cut in the 1911A1
3. trigger is shortened on the 1911A1
4. longer grip safety tang on the 1911A1
Here's another detailed explanation from M1911.org
From: Hickok45