its a finish semi-wildcat, a 7,62x54R Mosin with a 8mm S(8,20 mm diameter) bullet. a good solution if you have a ruined barrel but I believe the main reason was that that finnland was having a hunting law after WW 2 for a while that prohibited the 7,62mm for big game hunting.
same story like the 8x57IS and the 8x60 after WW 1 in germany. senseless law's at all

classic rifle, I believe Sako is allways making factory ammo in 8,2x53R