Here is a rare M 16 Latvian, that is not a German, or a early German, which I have never seen, exclude the Latvian one.
Firstly for comparition is Estonian M18, standard re-issue of a German helmet. Both helmets is 66 in size. (Latvian helmet still has vent horns as for size 64), Back neck guard appron on the Latvian helmet is more round by shape, and more solid by steel. All the rivets and chinstrap fastenings are red cooper made. Unfortunately that helmet has no liner, but it must be like a french M 15 Adrian type, and has sizes. Also the Latvian helmet is lover than a German