I have heard that story in several variations and, with due respect to our WWII vets, don't believe it, although by now the vet probably does. It was a way to explain to the family how that rifle captured in hand-to-hand combat with Hirohito got the crest ground off.
Other variants are:
- Japanese troops carried grinding wheels with them and removed the crest before a Banzai charge
- Navy men had grinding wheels ready when the soldiers/marines got back on the ships
- Officers stood on the beaches with files or wheels and removed the crests at the troops left an island
- Harry Truman kept track of every captured Japanese rifle and sent the FBI to seize them and grind off the crest before returning them
- All Japanese rifles were seized and ground in the states when the troops returned (the above version)
- Americans ground the crest because otherwise the Japanese would sue to have their property returned
I am sure there are some I have not heard or forgot. But the story is simple. The crest (mum) was ground by Japanese at depots in Japan before the rifles were turned over to the occupying forces for souvenirs or for destruction.