It should be interesting to see what the experts say about all these marks. But, basically the combination 84/Town is late 19th century leaving MK (cyrillic) as only one maker on this list:
https://www.925-1000.com/Frussia_makers_A_r.html
The 875 oval mark is Poland, a re-marking of standards...the shape surrounding is older, so maybe it is a transitional marking. I would suspect the the other letters are somehow related, but there seems to be arguments about Cyrillic vs. Latin as used in Poland over time.
That said, I am certainly no judge of Russian work (authentic vs. fakery) but there are many threads you can look at concerning this problem in Poland. I defer to the experts at this point. Hope that helps until better comes along.