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There are a variety of differences between the original Colt M1911 and Colt M1911A1 pistols.  The changes are illustrated in the below image.

Among the differences in the 1911A1 are a thicker front sight, an arched mainspring housing, a longer hammer spur, a shorter trigger, and a longer grip safety spur.

Differences between 1911 and 1911A1

Differences between 1911 and 1911A1

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