The Mum is the stylized chrysanthemum just to the rear of the pressure/gas relief hole. The " mum " is a royal symbol of the Emperor. Rifles that were captured in the field had the mum intact. rifles that were surrendered at the end of the war had the Mum removed or defaced. The Japanese used several types of rifles during their military reign. your rifle is a Type 99 ( or what is left of a Type 99 ). Yes, the stock has been sporterized, but the metal seems to be correct for the most part.
That is the short version of a very long story, hope that helps.