That looks like a spin off of the 20? 30mm? anti tank gun that came out just before world war two and was used by the fins? I cannot remember but I recognized the skid, the barrel brea, the top loading box mag and the trigger group.
axis of optical sight offset relative to the axis of the barrel to the left. Like on Boys antitank rifle, and another weapon with top placed magazine. Bren, Madsen, type 73 machineguns; boys, type97, lahti L-39 anti-tank rifles, and many others.