Bought a Franchi Affinity 20 gauge and a Marlin 336 mount to install a Vortex Venom. The Franchi is drilled and tapped but the two back holes on the shotgun don’t match the longitudinal holes on the mount. Also, the mount doesn't match the curvature on the top of the shotgun. It looks like it would rock even if I could mount the bracket on the shotgun. I thought this was the mount to buy?