The five differences between the M1911 and the M1911A1 are:
a) The tang of the grip safety is extended to provide better protection for the hand.
b) A clearance cut is made on the receiver for the trigger finger.
c) The face of the trigger is cut back and knurled.
d) The mainspring housing is raised in the form of a curve to fit the palm of the hand and it is knurled.
e) The top of the front sight is widened and the rear notch is also widened.
This comes from an old (perhaps '50s) US Army manual I have on the "Colt .45 Automatic Pistol".
