Regarding this particular P-51C/Mustang Mk.III – it was initially painted in standard USAAF markings, and then had RAF markings applied over them. From what I recall, the RAF considered the USAAF neutral grey bottom surfaces to be a close-enough match to the RAF medium sea grey to not have to be repainted – so I assume that the bottom surfaces were left alone while the upper surfaces would have received the standard RAF dark green/grey camouflage. As such, all of the USAAF stars & bars would have been painted over except for the one on the bottom of the right wing – and thus in that one and only location, the USAAF star & bars would still be seen, with the RAF roundel applied over it.