>Seemed like a confusing sort of
>nomenclature (at least from the outside), though I suppose that if you
>had the proper context, it would be less so.

M1 Abrams tank
M1 bayonet, as fitted to the M1 Garand
M1 carbine
M1 Combat Car, not to be confused with
M1 Armored car, which is not the above tank
M1 Garand, a battle rifle, onto which one would fit said bayonet, and which isn't the carbine
M1 Helmet
M1 mortar
M1 rocket launcher, a bazooka variant
M1 Thompson submachine gun
Mk 1 Underwater Defense Gun, an underwater pistol
M1 Thompson carbine, which isn't the submachine gun
57 mm Gun M1, also known as the British Ordnance QF 6-pounder
90mm M1 Gun, one of a family of dual purpose guns and not the above 6-pounder
120 mm M1 gun, an anti-aircraft gun and not the 90mm. OR the 6-pounder
240 mm howitzer M1, see previous bullets
M1 Super 90, a semi-automatic shotgun made by Benelli
HMS M1, an early submarine, noted for being equipped with a capital-calibre gun
M115 howitzer, also known as the "8-in Howitzer M1"
M1 flamethrower

There's a handful of others which needs a bit more of a stretch to 'fit' under the M1 name, but...

...!
Stoke Mandeville, Esq & The Victorian Ballsmiths
"Nobody Want Verdigris-Covered Balls!"