A new suspension system for the M1 Helmet Liner was introduced in 1964. The new system featured three webbing straps that could each be adjusted to hold the liner at the right height on the head. The liner also had a new neck band assembly, which consisted of a rectangular webbed body with three straps that attached to small buckles inside the liner. The P64 Infantry liner was also the first version not to have a leather chin strap.
Between 1964 and 1969 the new helmet liner was made from laminated cotton duck. However, the P64 liner was also produced in laminated high strength nylon fabric between 1964 and 1974. The nylon helmet liner offered improved ballistic protection and was 2 to 3 oz heavier than the cotton duck version.1
Liners were often painted and worn without helmets by various units in Vietnam, such as the Military Police.
1. Reynosa, Post-World War II M-1 Helmets, pp 42 and 49