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.
The same producer, Eisenhüttenwerke - Thale, this time with a ckl stamp. The change in manufacturer code from ET to ckl occurred during final production of the M40 and initial production of the M42 models. M42s can be found with ET stamps and M40s with ckl stamps. The change to an ordnance code (ckl) was to protect the identity and location of the manufacturer from the Allies. Notice the lot number «2938». The lot number indicates the particular batch of sheet steel that was used when a quantity of helmet shells were produced. This was accomplished through several steps of press-forming or hot-stamping the shells. The lot number serves as a control number for the manufacturer and as an extra stamp of approval.
Manufacturing stamp from the Quist factory, Q64. The number 64 represents the number of centimetres around the edge of steel shell. This number can vary from 60 cm to 70 cm (there are some examples known which are even bigger). The Quist factory primarly manufactured M35 and M40 models however an extremely small number of M42s have been observed.