Stocks and handguards had drawing #'s, so I'm pretty sure it's not a steel type. Here's what I found -
In Canfield's Collector Guide to the M1Garand and M1 Carbine, he states SA devised the system. Each letter (A-F) designated a "drawing size".
In Poyer and Rieschs' book on the M1, it says "Four sizes of drawing sheets were used, designated by the letters A (8.5 X 11 inches), B (11 X 17 inches), C (17 X 22 inches), and D (22 X 34 inches). Maybe those were the four used solely on the M1?
Didn't see anything in any of my '03 books that I browsed through, so, open for correction here, too!
Mike