Produced from 1964 onwards, the P64 Parachutist's M1 Helmet Liner was made from laminated high strength nylon fabric and featured the same improved suspension system as the Infantry P64 liner. As with the 1958 model, the new parachutist liner had two female snap fasteners, one on each side of the shell, which mated with the male snap fasteners on the helmet chin strap. Similarly, it was also equipped with an open web chinstrap that buckled to A-straps located on either side the liner. The new suspension system meant that four additional rivets were required to secure the two A-straps.1 As U.S. forces rarely performed combat jumps in Vietnam, Airborne soldiers often left the chinstrap to hang lose or removed it altogether.
1. Reynosa, Post-World War II M-1 Helmets, pp 99