I was recently given this 90mm M12 Drill cartridge dated 1944.
The headstamp is:
DRILL CARTRIDGE BASE 90 MM M12
The manufacturer (initials MMC) is:
Machined Metals Company
West Conshohocken, Pennsylvania
Thanks to Fede for identifying the correct manufacturer.
The materials of construction are curious. In TM 9-1901, Artillery Ammunition, 1944 (pages 165 & 166 below), the assembly was changed from bronze and brass to malleable iron at some point up to 1944 (the dummy fuzes can be made of one of several materials listed below). My cartridge appears to have a main body of malleable iron and the fuze is probably brass. But the base plate has the appearance of clad steel and brass. Possibly a mix of parts (brass/bronze, steel/iron) was used until old stocks were used up. The bottom of the base looks like steel about 3mm thick with brass above (See photo of rim below. Somewhat visible are the two layers that make up the rim itself [not the turned step, look at the edge of the rim]). Perhaps there is some other explanation for this (brass-plated steel?). I have not checked the various parts with a magnet yet, but the steel/iron parts are rusty.
Below is page 2-68 from TM 9-1300-203, Artillery Ammunition, 1967.
Below is the 90mm Anti-Aircraft gun M1A1 (I believe) from wikipedia (https://en.wikipedia.org/wiki/90_mm_Gun_M1/M2/M3)
Edited to correct manufacturer.