This is quite a rare helmet and the final hour for bidding is drawing near so I will list my thoughts on this so that others may benefit. Draw your own conclusions and feel free to poke holes in my ideas.
The owner has posted his fathers service record on E-bay: he began his service October 1, 1908 as a Einjaehriger with the Infantrie Regiment 115. He fulfilled his obligation on September 30, 1909. Returned to active duty August 4, 1914. Vizefeldwebel July 27, 1915, Leutnant December 14, 1915. The promotion from non com to officer might explain the extra 2 holes in the helmet. The record lists all assignments, times wounded and battle participations.
So at the time of helmet purchase he was a OYV which would explain the lack of a pierced crown on the helmet plate. It appears that there were studs on the cruciform spike base at one time so he could have changed to star studs when he was promoted to an officer. Why why are there two extra holes behind the helmet plate? These were not for ventilation as there is evidence of washers around the holes. So at one time there was another helmet plate.... Why? The ventilation holes in the spike are not aligned with the arms of the spike base in an east west orientation. The 1621 ribbon is fixed to the helmet with nuts and bolts but the bolts protrude into the helmet at least one half inch and are not clipped off by diagonals as normally seen. Ouch! The motto Gott Ehre Vatterland..... Vatterland is hyphenated. I cannot see the hyphen on the motto in this case. Lastly I question that the the motto is enameled ie glass. It may be enamel paint???? Not sure.
If this were real the helmet should sell for $8000 - 10,000 so we will see. The real killer in my mind is the extra holes in the helmet shell. I cannot come up with any explanation for this. Bill