M1D sniper rifle with M84 scope is shown above an M1C sniper rifle with M81/M82 scope.

The original rear sight of the M1 would not hold adjustments very well, so a locking bar was added in late 1942 which could be tightened after sights were set. This is shown in the M1C. An improved sight, the T105, was standardized by the end of WWII, and is shown on the M1D, above.