Military whatizit?

Ihave owned this Japanese bayonet for several decades, but have never known what the modification to the handle was made for. The blade, cross guard and scabbard are all normal, but the typical slab side wooden grips have been replaced with a rounded, well fit and finished handle. As such, it will not mount onto a Arisaka rifle. I am not sure if it was modified by the Japanese (though not usable on a rifle may somewhat rule that out), done as a piece of trench art by a GI in the field, or worked on in someone’s basement at home after the war.

Any thoughts on this would be appreciated.