I have yet to see an Arisaka that didn't look like it was drug through a swamp and put away wet. They were most certainly used, and used hard. There was a reason why Japan was cranking out "last ditch" rifles and arming civilians with bamboo spears. They really didn't have the guns to spare. They did put away a few meant to be used during the invasion of Japanese mainland that never happened. The fact that it has a mum tells me that it was most likely a captured rifle, not a post war surrendered rifle.
Actually most Type 99s were hardly ever used. I've yet to even hear of a Type 99 with a worn barrel. The early Type 38s tend to have been used hard, but not the 99s. Production of them didn't even really get on the way until realistically 1942 so the ones that actually were used in combat were used for maybe a month. Their round count would be very low.
The reason the stocks look the way they do is because of this:
They were all thrown into piles and GIs just picked one up from it.
For OP, it's a Type 99. I'm guessing Nagoya Arsenal of the 5th or 6th series. I can tell you more if you post a picture like what I posted below.
I collect Arisakas. I own 15 Type 99s alone....yeah I have a problem.