We have these in the Riga war museum (or very similar design). They are castle wall guns designed for two man teams. They came with these sorta reverse U shaped bipods one individual could hold while the other was behind the trigger. Designed to be super mobile, a castle or fort would have dozens of these moving back and forth along the tops of the walls.

Figuratively, the world's first 2 man sniper teams! But en masse for anti-siege defense.