It seems your helmet has seen some restauration and cleaning... You can still see green remnants on the helmet plate, that has been totally oxidised (green) and then cleaned up, in fact the complete outside of the helmet appears to be cleaned up...
The rear splitpen of the spike has been replaced once, the kokarden and chinstrap are in much better condition than the interior of the helmet, so they appear to be replaced also...
This type of chinstrap is normal for this enlisted men's helmet.
Maybe you can make some pictures of the stamps also?
Adler