That is exactly what it is..............Its a Japanese training rifle made from a GEW 88.
From what I read most had the magazine removed, and the sights replaced with Arisaka ones. Yours looks like one of the uncommon ones that still has the mag and 88 sights.
Its a shame the stock has been cut, Probably gonna next to impossible to replace....
I do not know if these trainers are safe to fire....
The Japanese would make training rifles from captured enemy ones.