this design is better to avoid air fittings blocking damper lock rings when screw it to the bottom. this design is better to avoid air fittings blocking damper lock rings when screw it to the bottom. this design is better to avoid air fittings blocking damper lock rings when screw it to the bottom. this design is better to avoid air fittings blocking damper lock rings when screw it to the bottom. this design is better to avoid air fittings blocking damper lock rings when screw it to the bottom. this design is better to avoid air fittings blocking damper lock rings when screw it to the bottom..syi