The Supermarine Spitfire Mk XVI was the same as the Mk IX in nearly all respects except for the engine, a Merlin 266. The Merlin 266 was basically the Merlin 66 but was built under licence in the USA by the Packard Motor Company. The "2" was added as a prefix in order to avoid confusion with the engines, as they required different tooling. All Mk XVI aircraft produced were of the Low-Altitude Fighter (LF) variety. This was not determined by the length of the wings (clipped wings were fitted to most LF Spitfires), but by the engine, which had been optimised for low-altitude operation. Almost all production Mk XVIs had clipped wings for low altitude. Many XVIs featured cut-down rear fuselages with bubble canopies. Some but not all variants had built-in extra fuel tanks in the rear fuselage. Because of a slightly taller intercooler and rearranged accessories on the Packard Merlins a new, bulged upper cowling was introduced (which also appeared on some late production IX’s).