[Oblongata]

I think the top helmet is actually a M1935 style helmet, produced from 1935 to 1940. Lots of information here:

http://www.german-helmets.com

[Little Bill]

Your right, when I looked at it I thought that the Stud was the face mask attachment on the WW1 helmet eliminated on the 1936(?), further back. The air vent shown was eliminated in 1942(?).

If you notice that the skirt of the helmet shortned as the war went on, probably feed back from the troops. One of the interesting things about the German Army on the Eastern Front was the high proportion of casulties from small arms fire as opposed to artillery on the Western Front. Looks like a lot of Prone firing going on and the skirt diging into the back of the Infantry Man.