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.