This does not seem like a military stamping. I have a lettered 12-gauge 30" Featherweight manufactured and shipped to the Rochester arsenal.
in 1941. It is stamped with a US Ordnance flaming bomb plus a square logo that reads "MP A Co." and "NP.
Interestingly enough, it also has a second set of 26" barrels numbered to the gun, presumably done by the factory after WWII. A unique item?