Looked at my halftracks. I have two distinct types.
Type I: is a sleeve (two pieces of steel clamshelled)that measures 12 x 16 inches
Type II: is a single sheet that uses the door as the backing and measures 11-1/2 x 16 inches.
the type I's are om my White 41 M2 and the Diamond T 44 M3A1.
the type II's are on the 43 Autocar M2A1 and the White 43 M13/16.