"Zinc-plated steel, for sure."
Not in 1942. At least it should not have been.
From Remington Arms (RA) or Frankford Arsenal (FA) where test batches of steel cased .45 ammo was loaded, yes, but from EC? I don't think they loaded any steel-cased ammo in 1942.
Steel case production wasn't fully authorized until 1943, and then only until the copper shortages that made steel cases a necessity eased, which it did early in 1944.
Evansville Chrysler and Evansville Sunbeam (who manufactured the cases for loading by EC) produced virtually all of the steel-cased ammo used during WW II, including quite a bit of .30 M1 Carbine ammo.
The steel cased ammo was never authorized for combat use, only stateside training.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.