Interesting, I haven't seen a sideplate that has those markings before, could be an Imperial or early Soviet one
Heres a reference for the various sideplate markings but I don't see yours anywhere
http://7.62x54r.net/MosinID/MosinRareNagant.htm
I asked my Russian-speaking friend to look at the website posted with the mark showing, and to give me a summary. Here's what he said:
"According to the article, this is an experimental model, made anywhere from the czar's period through 1930. Considered somewhat rare."
Wonder what was experimental about them? Looks pretty much like the other 2 I have. Obviously it has been arsenal rebuilt, so that may have removed whatever was experimental about it.
Maybe the unit it was assigned to was an experiment?
It was made by sober Russians. J/k, that's an amazing find!!!! Jealous!!