Strom67
There were 3 different models of steel helmet (stalhelm) during WW2, the number corresponding to the year of introduction, but actually the helmet goes back to 1916 and the M16 (plus two pre mass production models before that.)
There were 5 manufacturers during WW2,
ET - Eisenhuttenwerke, Thale (later CKL)
Q - Quist, Esslingen
SE - Sachsische Emailler- und Stanzewerke, Lauter (later HKP)
NS - Vereinigte Deutsche Nickelwerke, Scwerte
EF - Emaillerwerke A. G. Fulda
Hope some of that is of interest dennyboo!