Straight from TM-9-1901 (1944 and 1950):
Pages 165 and 166 of TM-9-1901 Artillery Ammunition 1944.
201. CARTRIDGE, DRILL, M12, W/FUZE, DUMMY, M44A2, 90-MM GUNS, may be stamped either M12 or M12B1. Both are functionally alike and completely inert. However, the M12B1, which is being manufactured at present, is made of malleable iron and steel parts whereas the M12 is made of bronze and brass parts. Both are 1-piece castings threaded at the base to receive the base plate and at the nose to hold the M44A2 Dummy Fuze. The base plate is held in position by a set screw; the fuze, by a set screw and shoe arrangement. Bothe parts are replaceable. FUZE, dummy, M44A2, used with this cartridge, may be made of bronze, brass, aluminum, copper alloy or sintered iron. In form, the casting resembles the M43 Fuze but is without the time ring. The complete assembly weighs 39.15 pounds, and is 37.44 inches long. This drill cartridge is not used in the 90-mm M2 Gun which has a power-operated rammer.
TM-9-1901 Artillery Ammunition 1950, page 187:
156. Cartridge, Drill, M12B2, w/Fuze, Time, Mechanical, M43A2, Inert, for 90-mm Guns M1, M1A1, M1A2, M2, M2A1, M3, M3A1, and T8
The M12B2 drill cartridge, which is the standard type, consists of a body of manganese bronze, a steel base, and an inert M43A2 fuze. Weight of the complete round is 42.04 pounds. Earlier models designated M12 and M12B1 (M12B1 is made of malleable iron) were assembled with dummy fuze M44A2 and are usable except in guns with power rammer. Malleable iron cartridges are not to be used under any circumstances in guns with automatic rammers. Bronze cartridges may be used in all of the above weapons. When used in the M2 or M2A1 guns, the power rammer will be made inoperative and the cartridge will be rammed by hand.