A defaced mum is the difference between a "batlefield pickup" and a "surrendered" gun.
Usually, the guns with defaced mums came out of collection depots, where GIs were allowed to pick one out and take it home with them.
On the other hand, a mum that has not been defaced often denotes that the rifle was picked up after a battle or taken from a Japanese soldier after a battle and kept as a souvenir, hence the relative scarcity.
Sadly, us kids who grew up in the 40s and 50s would often use these as playtoys, as Japanese guns in particular were not held in very high esteem. This accounts for some
of the more abused looking guns, as they were often repeatedly dropped when the kid who was playing the "Jap" would be "shot and killed" in ""battle."