This is an authentic M42 WW 2 German helmet. It was made by Vereinigte Deutsche Nickelwerke AG, Schwerte-noted by the stamp pictured.
It is possible this is a new previously unknown stamp since the N is not obvious. It was made in 1944.
It is a size 64. It does have the original chin strap, but it may be too short to connect properly. I have no idea if the stamp is NS64 or if it is a newly discovered stamp,'IS64' or something close to that? In any case the'N' is very poorly stamped making me doubt it is actually an N. I will leave that to you experts. The item "Authentic Rare German WW2 Luftwaffe Helmet M42 NS64 with strap old steel WWII" is in sale since Thursday, August 11, 2016.This item is in the category "Collectibles\Militaria\WW II (1939-45)\Original Period Items\Germany\Hats & Helmets". The seller is "millerconsignments" and is located in Charleston, South Carolina.
This item can be shipped to United States.