M42 (1942) Shell (non-rolled rim edge gives that away). More than likely an original shell.
On the inside left rim you should have a two or three letter code and a two digit number (eg. ET 64). The code designates the factory, the number the shell size. Paint might be covering it. The back inside rim should have a four digit number (batch number).
I believe that some of the original paint is still under the post war paint. If it was me, I would use warm water and an antique paint remover like "Circa" and a sponge to remove the post war paint. That way you have an original shell with original paint.
The liner and chin strap are post war military.
The decals are reproduction. Early in the war you had DD helmets (Double Decal). Branch on one side (Wehrmacht, SS, Luftwaffe, etc) and the other political (Nazi Emblem, National Emblem, etc). Off hand I want to say in about 1941 they moved to single decal helmets (decals applied at the factory and many shelaced at the factory they felt the political decals were too noticable (snipers etc.). However, troops could still buy the National decals at kit shops however and some troops still liked to purchase and apply them (I know a vet who liberated some national decals from a kit shop in 1945). SS were know to apply the Nazi political decal this from time to time from 1942-1945, a relection of their fanaticalism. However, an M42 with a double decal is always looked at with scruitiny as generally they are collector fantasy.
Just the shell might sell as high as $200-250 (repros cost $100) and I saw similar helmets advertised for much higher at a local gun show. All matching helmets go for $500+ in Canada, slightly cheaper in the US (more supply) and prices can go well over $2000 if a legit SS or Camo helmet. Helmets are highly faked and have been over the last 65 years. How do you tell an all original helmet from one that is original but had original decals applied post war 40 years ago?
As said, if I were you, I would strip the post war paint, look for an original liner (or at least get a good reproduction liner) and restore the helmet. You don't need to apply decals, many helmet lost or had none.
