The US M1 helmet replace the 1917 Brodie (Tommy Tin) in 1941 and 22 million produced by the end of WW2. The M1 helmet consisted of 2 parts outer shell and inner hard hat type liner. It was a one size fits all design that was used for over 40 years, until replaced in 1985 by the PASGT helmet. The M1 mainly only changed straps and coverings during its use from WW2, Korea, Vietnam and other conflicts.