Patrick,
This was quite difficult to read -- nonetheless, this is a repaired helmet visor. Both the F. Mark and the BJ indicator repair facility and quality control. It is quite possible that this back visor came from one helmet and was applied to another. The marking does look somewhat Bavarian but at times Bavarian units were in Prussian corps. So given that this was on an M15 helmet -- it is difficult to say where it came from. -- not much help here.