All German helmets manufactured before 1942/1943 have the factory code and shell size stamped on the inside near the left chinstrap bale (D-ring). Later production of M40 and M42 helmets
have the factory code stamped on the inside rear skirt above the lot number stamp.
Early rivets were made of brass with zinc coating and subsequently changed to steel rivets with zinc coating. Early production rivets can also be found in aluminum but only in small numbers.