An original American M1 helmet this being the rear seam variation, with swivel bales. Complete with Korean War era liner. Liner is made by Micarta and dated 1952 but also has a Westinghouse logo, so this liner is one made for the Korean War, produced using the Westinghouse World War Two moulds (which were sold off to other companies at the end of World War Two). The Woodland pattern helmet liner does not have any makers marks inside, but is likely Vietnam era.
Has been painted over inside, so no heat stamp visible.