Congratulations on a unique rifle brought back by your grandfather. A definate family heirloom!
In looking in my books, the one piece stock is standard for the Type 99 Naval Special Rifle. (The cracks in the buttstock are probably due to age [I can testify to that condition.] )
In "The Japanese Type 99 Arisaka Rifle", "The rifle design is unique in that the receiver is cast iron. The locking recesses for the bolt lugs are machined into the breech of the barrel so that the receiver supports only the bolt and magazine".
The metal was painted with black enamel, not blued.