WWII Paratrooper Helmets
The American Airborne troops used the same M1 helmet as the rest of the Army (and Marine Corps). However, the liner and chinstraps were modified to prevent them from separating during parachute jumps. These were known as the M2 and M1C helmets. The first paratroopers used the M-2 (a.k.a. "D-bale"). These had a modified, D-shaped wire for the chin strap on the shell. This was found to be susceptible to breakage, so it was switched in favor of the swivel bale design in late 1943.
The M2 was by far the most common paratrooper helmet on D-Day, but the M1C began to replace it late in the War.
Original WWII Paratrooper helmets are rare and very expensive today.