The reason for the same designation is that before WWII, the Army used the year of adoption as the model number. E.g. M1873, M1909, M1919, etc. It worked OK when the Army didn't buy much, but in WWI they had to buy a bunch of stuff in 1917 for the war, so there's alot of differen things that were "M1917". Obivously it causes confusion. When it became apparent that the world was probably headed for war, the US changed the system to the "M" model number. Hence we get things like the M1 Garand, M1 Carbine, M1 steel helmet, M1 ammo can, etc.
It's changed slightly to deal with prototypes, but we're still using the same number sequence today. E.g. the M4 Carbine is the 4th carbine (M1, M2, M3 previous).