Haven't seen one like that myself, but it's a windage-adjustable rear sight, and they were not available until the 60s as far as I know. The RPK has one.
It is a commercial lyman or williams adjustable peep sight that was added to the rifle; most likely by a previous owner. It is highly doubtful that it was added by the Russians, but it is an interesting modification. I have one on an old sporterized arisaka and have seen quite a few on sporterized mausers and 03 springfields, but this is the first one I have seen on a nagant.