The Colt 1911A1 was the firt major design variation on the 1911 service pistol. The M1911A1 was updated in 1924 based on feedback received from World War II.
The 1911A1 had a longer grip safety spur (aka Beavertail) and an arched mainspring housing among other features. No internal part or design changes were made. All mil-spec parts for the 1911 are interchangeable with the parts for the M1911A1 pistol.