Agree with Geno. Prewar, maybe put together from prewar leftover parts in 1944-46, no later.
The letter you refer to is a proper modern Cyrillic alphabeth letter, standing for one of the vowels, and consisting of two symbols. The stamp is actually the nitro proof sign, in the very literal sence, consisting of the words "proofed with smoleless powder", and it's this mark that dates the gun. The other marks on the right-hand flat mean the barrel dia. (15.5 mm), the BP initial proof, and the load of 4.8 gramms of (black) powder and 30 gramms of shot, which I think is the proof load (not sure, could be the recommended load for the heavy 20s).
The left-hand flat contains the indication to model, chamber length (70 mm), and the constriction of the barrel (the cylinder in the right, light full in the left were the standard setup for Model B, and in fact for all Soviet pre-War double guns). That is to say, you evaluated all the proof marks correctly.
I think it's exactly the case for "judge the gun, not the name" approach. The Model B was at a time the most common gun in the USSR, which naturally brought about a lot of legends. In reality they were - in the lowest grade - little more than overweighed clankers with greatly variable quality; of 10 guns, 1 or two would be as good as they get, 2 or 3 worthless junk, and the rest somewhere in between. Naturally, the best ones figured in legend, and the worst ones were quickly disposed of and forgotten. The higher grade guns were made in all levels of quality from good to best. The factory was really capable when they wanted; in the early 1920's they made an exact copy of the Westley Richards drop lock system, and it turned out not much worse than the original.
This gun is surely above the ordinary, by how much - I can't tell, the engraving is just what you'll see on the lowest grade guns, the stock looks as good as the highest grades. I think you'll do a reasonably accurate approximation of the value if you think of it as a Belgian gun of the same era, with no name recognition factor.