I picked up a really nice Type 38 Carbine at Tulsa this year with similar interesting damage. Same thing; all original, even (most of) and original sling! And several gouges taken out of the grip and lower tang metal. My first thought battlefield/shrapnel damage. It was actually kind of a selling point for me; its got a story to tell, if only it could speak (and if only I could understand Japaneseicon). I'll post some pictures later.

I think you have a really nice carbine with character!