The number in the first photo of these new ones is your FSN or Federal Stock Number. Most of them have a date in the FSN. Yours and I could be wrong would date from 1975 the very end of the war. Your chin straps on the M1 helmet are from Korean Era to Mid Vietnam era.
Sir: The M1 Helmet as it sits on a head is actually a M1 Helmet System. It consists of 2 parts the Outer and the liner. The outer is Referred to as a Shell or Steel Pot, brain bucket, stew pot, pisspot, etc. Most folks refer to it as the M1 steel pot.
The second part is a liner made by numerous mfg'rs. It consist of a M1 shape fiber/paper outer section and the straps inside that keep it off of your head. There are also a chin strap on the liner and another on the Steel pot.