XJS type alloy are fitted from 1981. Trip computer 1983-on. Taller gear ratios, 3.07 and 2.88, instituted trying to meet CAFE requirements. Ventilated rotors on front brakes from 1974-on. Brake and suspension components virtually identical on all XJ cars since 1974.
All models have Lucas electrics unless otherwise noted.
V12 cars featured positraction rear axles. There are exceptions to everything. All 1981 XJ6 Sedans fall between 320092 and 330272. Your VIN is important because it is the final authority on when your car was built and what components apply to it. Keep you VIN number handy when ordering parts for your Jaguar.