It was easy to tell its an IX, the fishtail exhausts (the V and earlier had 3 on each side, the IX was the first to have an exhaust per cylinder), and the elongated nose (considerably longer than the V), failing that there is always the REALLY big clue of two under-wing radiators, earlier models had one and a cylindrical oil-cooler

Mark (lover of all things Merlin and Supermarine, needing to get a life)
