I found this key tag in my grandfathers stuff. He was in active duty overseas. He would not speak much of war, so I know little about this. I believe it has something to do with the big guns like on the Bismark. It`s an aluminum disc 29mm in diam, 1.5mm thick and has German words stamped in.``Verbrennungs-raummesser`` on one side and on the other side``fur 38cmS.K.C/34. I would appreciate any help with info on this.