Gents...my M1916 steelhelmet mm "W66" not a common mm to be found on Imperial steel helmets, this mark is for the maker "Hermann Weissenburger & Co" of Stuttgart-Canstatt.
On the inside dome of every WWI German helmet you will find a heating lot code. These codes were used by the factories during production. In many cases the heating lot code will indicate where the steel was milled. These steel mills are called rolling mills. My helmet is marked "E42" which is sadly an unknown mark ! Most of Weissenburger & Co made helmets I have seen/examined have been marked with an "R" indicating the rolling mills at "Stahlwerk Röchling" at Volkingen, but not mine. It also has a black stencilled "57" which may indicate a depot refurbed helmet.....it has a typical 3 pad leather liner, behind each pad is a tied on pillow of horsehair or gauze.
Prost ! Steve.