If you're after a WWII era M1 helmet, I'd suggest that you seek out the following features:
1) Front Seam (change over to rear seam occurred in 1944)
2) Fixed Bale (change over to swivel bale occurred in 1943)
3) Khaki webbed liner (post war liners are webbed in dark green)
4) Cork based olive drab paint (post war paint is typically sand / silica based)
5) Green painted buckle on leather liner chinstrap (later war / post war was painted black)
6) Brass chinstrap buckle (opposed to steel - not to be confused with the liner chinstrap)
7) Schluter vs McCord helmet shells (there's a heat lot number on the inside when you remove the liner & Schluter helmets can be ID'd by the S)
8) Various wartime liner manufacturers include: Inland, MSA, Westinghouse, Capac, Firestone, Seaman Paper Company and Int'l Molded Plastics Inc.
I've really only presented the tip of the iceberg when it comes to this subject, but if you google the above noted terms, lots of additional info will be available. My suggestion would be to avoid unit markings / rank insignia in the beginning as none of that stuff was factory applied, so they're heavily faked. Read a little bit before getting into the more complex areas of M1 helmet collecting as mistakes can be costly.
Cheers,
SoH
