I always thought that the RAF designated it as the Mustang as at the time (as is now) aircraft were named rather than given a prefix (for example the Pavina Tornado, Sepcat Jaguar , Hawker Hunter , EE Lightning , Vickers Viscount , Hawker Hurricane , Supermarine Spitfire ....ect. ) and that the USAAF adopted the name after they realised that it was a good plane and stopped poo pooing it .