The U.S. Army used the M-[date] system until 1925, and the Navy used it until 1944.
Afterwards the Army switched to M-[numerical] and the Navy switched to Mk-[numerical].
There’s a lot of nuance and “well yes, but no.” edge cases, but that’s the simple explanation.
That’s why the Army went from the M1903 rifle to the M1 rifle. And why there were so many M1 items in WW2, as the new system had not been around for long enough to see a lot of turnover from M1 items.
The U.S. Army used the M-[date] system until 1925, and the Navy used it until 1944.
Afterwards the Army switched to M-[numerical] and the Navy switched to Mk-[numerical].
There’s a lot of nuance and “well yes, but no.” edge cases, but that’s the simple explanation.
That’s why the Army went from the M1903 rifle to the M1 rifle. And why there were so many M1 items in WW2, as the new system had not been around for long enough to see a lot of turnover from M1 items.