What is the difference of the m1 helmet from ww2, korea, and Vietnam ?

I know in ww2 they made 22 million helmets both front and rear seam. From my understanding the helmets made in ww2 were used all the way to vietnam, is this true? We're there any productions of the m1 helmet after ww2? If so how did they differ from the ones in ww2? Did they look different, or could you hardly... show more I know in ww2 they made 22 million helmets both front and rear seam. From my understanding the helmets made in ww2 were used all the way to vietnam, is this true? We're there any productions of the m1 helmet after ww2? If so how did they differ from the ones in ww2? Did they look different, or could you hardly tell them apart ?
Update: I'm talking about the shell not liners
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