It's an 1899 Izhevsk M91, on an 1899 Izhevsk receiver, that's been converted to a trainer by the Japanese. I'd bet my Mosin collection it was captured during the Russo-Japanese War. Otherwise it wouldn't still have the pre-1908 rear sight leaf. Certainly it could have been swapped with another capture, but considering the date and the known history of Russian and Japanese interaction that is the most likely case. Here's a list of the modifications I've discovered.
1. Rechambered to 6x5x50 Arisaka, but the bore is still .30 caliber. Somebody here, probably DocAV, once said they did this to fire blanks so the bore didn't matter.
2. Hex receiver reshaped to round, recoil lug cut off, and tang shortened (why?).
3. Rear sight base turned backwards and notched to hold the rear of the handguard. I'm sure the Japanese did this.
4. Barrel shortened and new front sight installed.
5. Solid floorplate welded in the feed opening.
6. Interrupter removed and ejector reshaped to eject the smaller 6.5x50 case.
7. Extractor channel reshaped to allow the extractor to catch the 6.5x50 case. The bolt face itself is unchanged and a 7.62x54r case still fits on it. The extractor just comes closer to the center than normal.
8. Complete new stock, handguard, nose cap, barrel band, and trigger guard.
9. Barrel turned down to accept a T30 bayonet, but the bayonet lug is broken off the nose cap. Someone cut the front sling swivel off the barrel band also.
There are some Japanese characters painted on the buttstock, but they're very faint and I can't read Japanese anyway. The seller says that they say "#5" The whole bottom of the butt is spiced on with a couple of dowels. The wrist was broken completely through when I got it but I glued that back together. You can still see the crack. It's no wonder as all the force is on the tang bolt which is just screwed into a nut set in the bottom of the stock underneath the trigger guard. I guess they figured blanks didn't need much, but if the rifle was used at parade just banging the butt on the ground would put a lot of stress on the stock.