Ron don't sell yourself short. Good train of thought.
I think you have 2 different marks. The easy one is the one on top B.I.A. VII Bekleidungs Instandsetzung (repair unit) VII Corps. I believe this is the last one added. The lower mark from 1916 he could be one of many things. Unfortunately, I cannot read what ever markings exist between the E and the A. The XV obviously means it is part of XV Corps that originally issued the helmet. I do not think it is a regular BA Mark from XV Corps. I think rather that it is an Army Corps level unit. The letter A could well stand for Abteilung, a rather tricky word that translates both to Battalion or section depending upon the unit or branch. The 1st letter-E-I would think stands for Ersatz which translates roughly as replacement. What is in between could be any number of things. This is just an opinion. Any kind of holes poked in this are fair. Is the helmet artillery?