The parkerized sheet metal piece is the M3 breech shield. It snaps on to the receiver. It was used when firing blank cartridges to keep any blow back particles away from the face of the shooter. You have the third version of the M12 blank firing attachment. It was made by either Hesse Machine & Mfg. Co. or East Moline Metal Products Co. Check the silver colored portion for a manufacturer marking.
From the canon,
"Additionally, the M12 BFA tube will not fit all the way into a T44E4 or early M14 flash suppressor. The early M14 flash suppressor, part number 7267088, did not have an inside diameter large enough at the rear end to allow the M12 BFA tube to be fully inserted. Springfield Armory drew up a new flash suppressor design, part number 7791053, in September 1960. The inside diameter shoulder at the rear of the suppressor was done away with to allow use of the M12 BFA with the flash suppressor."