 Other alterations to the wings included new navigation lights, mounted on the wingtips, rather than the smaller lights above and below the wings of the earlier Mustangs, and retractable landing lights which were mounted at the back of the wheel wells; these replaced the lights which had been formerly mounted in the wing leading edges.The engine was the Packard V-1650-7, a licence-built version of the Rolls-Royce Merlin 60 series, fitted with a two-stage, two-speed supercharger.The armament was increased with the addition of two more .50 in (12.7 mm) AN/M2 "light-barrel" M2 Browning machine guns, the standard heavy-calibre machine gun used throughout the American air services of World War II, bringing the total to six.