A comparison of the M1 and M1C helmets. The shells are both the same, the only difference is the chinstraps. The airborne chinstrap has an extra length with a snap on it that snaps into the liner, its meant to hold the liner and helmet together during a jump.
First is an outward comparison, take the net and scrim off and they are the same other than finish and color.
Once again a standard M1 Liner:
Now the airborne liner, you will notice there are some differences:
1. There is a set of canvas straps "A-straps" that are attached at the suspension in the front and the nape straps in the back that are used in conjunction with the chincup to keep the liner on your head.
2. The snap for the helmet straps is visible (unsnapped) at the top and is snapped on the bottom. Obviously these had to have holes drilled for them then the snap is set in.
Both the liner and M1C helmet are original WW2 but we had to modify the liner for the airborne stuff so we used a repro set for the suspension, sweatband, chincup, and other hardware. Ironically the brown leather chinstrap (which both helmets share, its wrapping forward over the brim on both helmets) on my M1C is original. It shows its age too.