Germany - Stadt Duren w/Helmeted Soldier & 'SD'

Here is the Stadt Duren version of the helmeted soldier, here with an "SD" in the field. A similar coin, was issued for Weissenfels (50 Pfennig 1918, see at this /photos/weissenfels-wwi-soldier-50-pfg-2259/ ). The major difference in the portrait is that here the metal stud is missing, I believe all German WWI helmets had this stud on both sides of the helmet.