I think it's an outboard mount Hispano-Suiza of one variant or the other (Mk.II?) out of the wing of (probably) a Spitfire or a Tempest. Round thingee sticking up is the belt feed. The tube looking thing is the mounting sleeve/housing from the wing, still with the leading edge socket. They welded pivot points and a handhold onto the sleeve.
Shows the ammo...note the long projectile, minimal necking of the Hispano-Suiza 20mm x 110 round