Better picture of rifle
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Better picture of rifle
Barrel must have been a pretty good one to keep on that long. Russians changed out the rear sight, then the Finns restocked it and cut the chamber for the D cartridge. Can't see any reason that the receiver wouldn't be original, but should be easy enough to check the date on the underside of the tang.
The nutshell is the Finns re-barreled many Nagants [M91/30 and M39?] with .308" bores, marked with a “D” on the barrel.
They built them as match rifles, but they shot their standard 7.62x54R [match?] ammunition through them.
Here's a thread explaining more... 7.62x54R and .303 bore - where it came from? - #7 by BlueBoy1969 - General Ammunition Discussion - International Ammunition Association Web Forum
As Jim says, the Finns did initially rebarrel M91's to .308 when the bores had worn out too much. Others, like yours, were still serviceable, and were left alone. In the 1930's, they began to use .310 bores in their replacement barrels so as to make use of captured Soviet ammo, including machine gun fodder. Their new heavy bullets were called the D166, and D marked barrels indicated that larger bore diameter. Your chamber's throat might not have even needed to be opened up at all, but was stamped to lessen worries about using any ammo available. Many run of the mill captured 91-30's are D stamped.