My guess would be from an American airplane.
Maybe strafing a target or a bomber defending itself or a fighter trying to shoot down a Japanese plane.
Probably, and most likely. But I don’t know if the Mitchell ejected shells, or what. The B-29s weren’t armed as far as I know. The local museum didn’t have a good explanation either. If it were from airplanes that ejected shells, you’d think there would be hundreds of shells in the same area, but we only found three.