As with so many things military, no rhyme or reason or consistency, depends on the era, I suppose. When the Army altered the M1903 at TR's insistence from the rod bayonet to the more practical M1905 bayonet the designation M1903 was not changed, nor was it changed when the caliber was changed from 30/03 to 30/06 in 1906. The M1903 modified for the Pedersen Device was designated the Mark I. The original designation for the M-1 Garand was US Semiautomatic Rifle, Caliber .30, M-1. When the gas trap was dropped in favor of the more practical gas port design-no change in nomenclature. The changes in the M1911-shorter trigger, arched mainspring housing, change in the frame cut for the tip of the trigger finger, e.g. -were so minor that you could easily convert one to the other. Conversely the differences between the M3 and M3A1 Grease Guns-dropping the cocking handle in favor of a simple finger hole in the bolt,e.g.-led to the A1 designation.