I was under the impression that the rifles / carbines with the ground-off "mum" were end-of-war "surrender" items rather than "battlefield pickups". It seems reasonable to assume that the Japaneseicon Quartermasters were not prowling the two-way rifle range with angle grinders, defacing the markings on the weapons of their fallen comrades. Of the Arisakas brought back by my Army Engineer uncle, from a 1940's tropical sojourn in New Guinea, none had been "defaced".

As for "colour", didn't the Japanese treat their woodwork with Potassium Permanganate (Condy's Crystals) as a mould / rot preventative prior to the traditional oiling? That might explain the peculiar "reddish / purple" colour of some of the nicer surviving originals.