Its a WW1 german shell. I think M-16?
Yes, an M16, the original German Stahlhelm introduced in 1916. Looks like a good one to me with original paintwork and liner band. I wonder if there might be a trace of camoflage paint. ET64 is the manufacturer (Eisenwerke Thale) and size marking. There is sometimes also a batch number.
On my one its in the dome in the middle.
Model 1916 with the metal liner band, commonly called the M17. As said, batch number is on te inside top of the dome. Hard to read is there is heavy dirt or rust buildup. Looks like a nice shell.
You are correct. In the dome of the helmet is a number "? W 3 1 9" I think there are two letters but I can't make out the first one. The second is definitely a W followed by a 3 1 9 spaced out quite a bit. Does this help?
Nice Stahlhelm M16 sometime called M17, for the iron part of the liner.
R.....Stahlwerk Röchling, Volkingen.
B.....Stahlwerk Becker, Kiefeld.
Hope that helps.
Prost ! Steve.
"The German Army is the perfectly adapted, perfectly running Machine. The difference is that the Germans are organised with a view to War...with the cold, hard, practical and business-like purpose of winning victories."
G.W. Steevens - The Daily Mail (1897)
Very nice M17, my favorite helmet :-)