I will likely take heat on this, but I don't think there is any "mystery" about the ground "mum". The Japanese were allowed to grind the symbol before surrendering rifles that were stored in depots or surrendered in mass after the end of the war. Those rifles were then passed out to about any American service member who wanted one.
An unground "mum" would almost always indicate a rifle picked up in a combat zone (as noted above, few Japanese rifles were actually surrendered). The person who brought the rifle back may not have been the person who "captured" it, as those rifles were common "trade stock" among Army, Marine and Navy personnel in the Pacific. Further, no combat soldier or Marine could carry two rifles while fighting a war, so support troops and shipboard personnel ended up with many of them.
But if a soldier or Marine got a "ground" rifle at the end of the war simply by taking one or having it handed to him, he might not find it impressive enough to state the truth. It would be better to tell how he wrestled it from a Japanese soldier in a life or death struggle.
Of course, if someone suggested that a ground "mum" would not be consistent with the capture story, the now-veteran might just embellish the tale with a story about how the doomed Japanese ground off the crest, or how the Navy removed it on the ship, or how he mailed the rifle and the Post Office ground it, or how the FBI came and made him remove it, or... You get the picture. As any politician knows, you never let the facts get in the way of a good story.
Jim
