M1 helmet shell

[denfly]

A friend of mine was cleaning out his garage because he's moving down south and gave me a M1 helmet shell with a manufacturers stamp 1 1821. Now Iv'e known M1 helmets to have either a 3 or 4 digit numbers on them but 5? Can anyone explain this? The first digit is away from the 4 digits. Please don't ask for pics. Either you know what it means or not , don't make speculations. It is the same format as 3 and 4 digit styles. My camera can't take such close-ups. By the way, it's a rear seam.

[reneblacky]

OK NO Speculation

[CSM Gray]

I is for the company Ingersoll the shell is anywhere from Vietnam to 80's era.

[denfly]

Thanks CSM G. So the 1 is actually an I. It certainly looks Vietnam era to me. I'll find a Viet era liner and sell it. Thanks again.