It is a Bulgarian National Banknote from 1951, 3 Leva. Here is one selling on Ebay. This auction lists it as Soviet Union Bulgarian. And this auction calls it a "Bulgarian National Bank Note," as does the PMG slip that contains it, so I think that settles its identity.
The communist Bulgarian government imitated a lot of Soviet things, including their money. I have some Bulgarian notes from 1974 that resemble the notes being used in the USSR so closely that you could probably pass them in each other's countries.