 Rolls-Royce engineers rapidly realized equipping the Mustang with a Merlin 61 engine with its two-speed two-stage supercharger would substantially improve performance.The company started converting five aircraft as the Mustang Mk X.Apart from the engine installation, which utilized custom-built engine mounts designed by Rolls-Royce and a standard -diameter four-bladed Rotol propeller from a Spitfire Mk IX, the Mk X was a straightforward adaptation of the Mk I airframe, keeping the same radiator duct design.