Here is a complete list of WW1 German steel helmet manufacturers. The list includes the known shell sizes made by and limited to each manufacturer. It seems that Eisenhüttenwerke Thale was the only ironworks that made them in every size.
~Hans
German Steel Helmet Manufacturers 1916 - 18:
Code, Maker name, location - (shell sizes made)
1. E.T. - Eisenhüttenwerke Thale A.G., Thale /Harz - (size 60 through 68 shells) Premier maker. The only manufacturer of the first model 1916 helmet (square dip) and the M18 ear cut-out.