No such thing as a "last ditch" Japanese rifle. There are substitute standard rifles which do not have all the bells and whistles that the earlier rifles have and as the war progresses, the bells got smaller and the whistles quieter but one thing that did not happen is a dangerous rifle. If it has the mum, it's as safe as any early rifle.
The confusion occurred most likely because Japan unlike most other nations, used training rifles which weren't real rifles. Well, a few of them were, old, worn out, etc but a lot of them were just put together out of whatever materials they could find and they looked like rifles. A GI not knowing that would take the training rifle home as a prize and attempt to shoot it which very probably would result in disaster.
The Arisakaaction is one of the strongest ever made and that mum on the receiver means it was accepted into the military as a fully functional and safe rifle.