Nice helmet Amy! I really like M15's, as probably all members here know.
But you know, I also saw the BJA VIII stamp, with the date 1918 (f) on the other side of the FsAR 18 stamp.
It might be, that the helmet got refurbished by the Bekleidungsinstandsetzung amt VIII, and was reissued to a Prussian infantry regiment, operating as reserve, behind the frontlines, in 1918. And thus, the spike could have been added by them. (BJA VIII)
It looks better with the kugel though!