The two models are referred to as trench guns and riot guns. Trench guns were meant for combat, while riot guns were for guard duty. Prices for the trench models have escalated faster than prices for the riot guns, at least partially because of the M12 trench gun's combat history with the US Marines in WWII. Both were made in two finishes, the highly polished commercial finish until late 1942, and the much more dull blued "war" finish later. There's solid evidence that some wartime US M12s were originally parked, as well.
You mentioned the obvious difference between the trench and riot M12s, the presence of the bayonet mount and perforated steel handguard on the M12 trench gun. The difference goes deeper than that. The trench M12 has a straight barrel taper. The riot model has the taper of the sporting M12 barrels, which is not a straight taper. Winchester made M12 bayonet mounts that fit the trench gun's straight taper only. The mount is held on to the barrel with bolts that engage three notches under the barrel. When these bolts are tightened, the mount fits the barrel solidly with full contact for its entire length. It will not move under recoil. If an attempt is made to install an original mount on a riot or cut down sporting M12, the mount engages the barrel only at the front and rear, and the stability of it is weak.
There's a company called East Taylor that makes nice in-the-white steel reproduction M12 bayonet mounts. IIRC, these are made specifically to be mounted on riot and cut down sporting barrels, and take barrel taper into consideration. They are not as well made as the original Winchester mounts and are not marked with a W, but may be just the ticket for someone who's converting a sporting M12 to trench gun configuration. The same company makes bayonet mounts for the Savage 520 and 620 trench guns, as well.
Original Winchester spare trench M12 barrels have dried up entirely, and basically can't be found in any condition.
Hope this helps!
Edited by TSMGguy, 13 July 2020 - 06:55 AM.