IMO, the serial number is too low for it to have been a US Military contract riot or trench gun. The Model 12 riot guns are mostly in the 970,xxx serial number range, and the serials for the trench guns are even higher.

All the Military Model 12s are factory marked on the right side of the receiver under the ejection port with a "US" and flaming bomb. There is also a small hand stamped flaming bomb on the top of the barrel next to the Winchester proof mark. The Military guns also have buttstock marks. The vast majority of the riot guns have a WB mark and crossed cannons mark on the buttstock while the trench guns have GHD stock cartouches and crossd cannon marked buttstocks.