Got this from a neighbor when I was a kid...I always assumed it was an old US helmet.  Looked at it today while visiting my mom...sometimes it's good to be wrong.

 

 

Based on the holes I think it might be either a Soviet SSh-39 or a Czechoslovakian Vz. 53 except that neither have the low hole on the back, I am not sure if any country made a helmet with a rivet there or if yours was modified at some point. 

By modified did you mean "hole drilled to hang on a nail"?

 

 

Yeah that'd be my guess

Not surprisingly there is no ink stamp to be found on this one.  It looks to have been used to test paint from spray cans at one point.