Pic of old finn

[Low & Slow]

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.

[Chainsaw311]

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.

Excellent info,thanks!