It's very possible that this is a pre-WW2 or very early 3R blade that had the original pommel substituted with the 3R pommel to meet the 1938 standards.It's not uncommon for these newer pommels not to tighten fully if they weren't factory refitted.Why it only turns 1/8th of a turn,I'm not sure.I've seen some daggers that had some sort of filler added to the pommel threads,possibly to tighten them up?The filler seemed to be a very fine sand,like the type used in making molds.Perhaps this is what is causing the pommel to jam? As to what the grip is made of,I suspect it's solid 'trolon'.I've only three Navies and one is an orange gripped example by WKC with hammered scabbard.Of my 50+ Heer daggers,only the ones with white grips have plaster or wooden cores,all the colored ones are solid(but I also have a few solid white ones).