Is it possible that stock carvings relate to Civil Guard districts? Not necessarily the "X" above, but I have an M24 with an "H" and it was used in the Helsinki district. I think I have at least one more Mosin with a similar carving.
One thing that jumps to mind - the Finnish Jaeger (Light Infantry) shoulderboard insignia consists of a bicycle wheel with a pair of ski's across the wheel. Could almost look like the ownre had intended it to indicate this