Used driver side (left hand) front drum backing plate for 2-1/4 inch brake shoes on the Mercury Cougar, Ford Mustang, and Fairlane / Torino with the 289 or 302 engine. Typically when the car can with the small block V8 they came with 10" x 2-1/4" brakes shoes.
Ford part # C3OZ-2013-B.
*NOTE* Normally 6 cylinder Mustangs came with 9" x 2-1/4" drum brakes on the front.