Rino,
IMO, the OR shell has been polished out. You are correct in that some JzPf helmets are nickel bodied, but those are officer interim helmets. I’ve never seen a nickel OR helmet.
Cheers,
Dennis
Steve, I do not own a copy, but I remember when it came out I considered buying one. They have a couple used at ABE Books
Dienst- und Rangabzeichen der offiziere und Beamte der Königlich Bayerischen Armee 1806 bis 1918.
Seibold, Walter; Schulz, Gerd M.