Good Morning Avs
I knew I should have done a better job with this, or maybe just not have said anything. I apologize. Please remember, I don't claim any expertise in makers' marks. I may be embarrassingly wrong about this but heck, I've been embarrassed before.
The stamped MM (if that's what it is) above the large oval/56 is the maker's mark, right? Not the bust and 56. If so, what appears to me to be an MM above the large oval/56 would be the maker's mark. I can't see it clearly because of the glare and it's not what you focused on, but that's what it looks like to me. It certainly could be something else.
I think of a classic M as two vertical lines with a V-shaped line connecting them at the top. However, there are stylized versions of all the letters and I thought a stylized version might just be a straight bar between the vertical lines, looking sort of like a Latin HH. However, I can't find such a mark in my (partially destroyed) reference book.
So, believing it to be a rather formal, standard MM maker's mark, I found the maker I mentioned. I couldn't find a (Cyrillic) NN makers mark. The only similar MM mark was mid-19th century so I eliminated it from consideration.
My book doesn't have a period for this maker, only a year - 1898. That puts it close to your time frame. If I'm reading the reference correctly, he was an assistant to Danilovski.
Good luck with your research. You can do much better than me.
Chuck