no worries mate
original WW2 versions are very hard to get hold of & when they do come up they can go alot of money its the one part of the M-1 that ive never been able to replace because of that !
If you search documentations of the US M1 helmet, (french or english) :
Régis GIARD & Frédéric BLAIS
Histoire & Collections
5, avenue de la République
F-75541 Paris Cedex 11
Histoire & Collections - Livres et revues
My opinion: good book, but not complete
And a SUPER documentation book in french AND english, but only for M1 helmets with fixed ball :
of the WW2 US GI
by Pieter Oosterman
I didn't know the straps had markings on them sorry. The nape strap has a bunch of numbers on the top and below that USMC-1945. I love that it has USMC, a little plus for me! The sweatband has POWELL and below that 1942. Is this a set or different straps to complete a liner. Different dates?hmmm. and forgot to mention the liner is a westinghouse. a circle with a W in it and a oval below the W. Thanks Chad what would the value be for this helmet also. I paid $115 for it. im happy though!
A '42 dated headband in the condition yours is in is very hard to come by and is easily worth what you paid for the entire helmet and liner.
What did concern me was you stated that the nape strape was marked USMC. I do not believe there were any marked specifically to the USMC. Please check agian and see if it might be marked S.M.C. instead. I attached a photo of what it may look like.
The nape on the photo is a "Scholl Manufacturing Company" fabrication, with zinc coated steel and blackened brass snaps bearing number "W11-009 qm 16874. A batch nearly 4 milion napes contracted on october 28, 1943.
I have 2 sets of engineer, or sapper wire cutters, man they are not cheap, but one is army in a green pouch it is marked U.S. & 1944 on the pouch & he cutters, but then I have a blank brown pouch, but the cutters are marked U.S.M.C., when I asked the guy why there was no date on the pouch, he told me that the marines didn't mark anything until 1944? don't know but thats what he said.