I have several unusual Mosin Nagants. One is a New England Westinghouse M91. I also have a Chatellerault and a Polish Oval 11. My Finnish Mosins range from SA, SY, SKY, S, SAT and VKT stamps. Each one has its own different and significant stampings. There's an online catalog that I know you're aware of due to your avatar, I'm sure you're also aware that the Star anchor wreath and ribbon stamping you show is not anywhere to be found on that catalog. To me this means one of 3 things. It simply falls into their "Unknown" category, Considering over 128 million Mosins(that's total production of all models worldwide) were made there's probably no true way of knowing every single stamping that was done.
It could also be a Unit Stamp that members of a certain unit had made for them, or they had an engraver do it for them "unofficially" for each rifle in that unit.
Lastly, "trench art" is a possibility, although it does look too refined for that, the common Russian soldier was conscripted from all walks of life, mostly peasant but affluent and educated were also drafted. It's possible a jewellers son made his way into the ranks of infantry and left his mark on his rifle.
Regardless, it's interesting, it's very unusual, and it's another mystery in the history of the Mosin Nagant rifles.