'First P-51 Flown by Leroy Gover' - AM121 was a Mustang Mk X, a two stage Merlin 61 engined Mk 1 conversion, works carried out by Rolls-Royce Flight Test Establishment. AM121: This aircraft arrived at the Rolls-Royce Flight Test Establishment at Hucknall on 7 June 1942 and was the first to be delivered but the last to be converted. A broader chord fin was installed but the aircraft was not slated for testing at Hucknall and instead was sent to RAF Duxford before being loaned to the 8th Fighter Command USAAF at Bovingdon along with AL963.