“The nicest Mosin versions are Finnish—gun’s seized from Russia by Finland in WWII, and re-barreled and re-stocked and named the M/39. The Finns make the best rifle barrels in the world.”
I suppose some were re-purposed Soviet weapons, but most were made by the Finns: http://www.mosinnagant.net/finland/finnish_mosin_nagantm39.asp
The Finnish Mosins all have Russian-made receivers, the stocks and barrels are Finnish-made. I say “Russian” as the Mosin-Nagant was a Czarist-era design, going back to the early 1890s—and many of the M/39 guns have pre-revolutionary receivers.
The Finns defeated Stalin’s troops in the Winter war of the late 1930s. In doing so, they were able to seize hundreds of thousands of Russian rifles. They didn’t like the barrels & stocks, but the receivers were solid, so they rebuilt the rifles to their own specs.