SO # is stamped on the rad support upper yoke near the driver's side, and should match the SO # on the fender tag. There's another # under the trunk weatherstrip on the left side horizontal surface towards the rear.
Also look for 11" drum brakes, dual exhaust hanger brackets, holes for RR nameplates on the doors, trunklid & dash; might still have "F70" written on the underside of the trunklid. No console brackets, no extra holes for bucket seats. Coupe only until mid-model-year. RM21 (coupe) or RM23 (hardtop) VIN prefix.