Germany Bodentruppenhelm 1A1, sub model M62
This helmet has on top something that looks like a button.
At the inside the button is locked with a nut that holds the liner in place.
You can see two rivets on the side of the helmet, these are there to hold the chin strap.
I found the following insciptions in the liner:
SW- ? / 73
The first number is the NSN or Nato Stock Number.
The second line shows SW this stands for Schubert the 73 is the year of production 1973.