Robert Grinsell, who has authored a number of books on the P-51 family, states that the only aircraft to carry the "Apache" name were the two XP-51's given to the USAAC by the British. The time line of events fits his assertion, the two XP-51 aircraft for the Air Corp were inserted into the contract on 20th September while the letter from the British naming the aircraft "Mustang" is dated 12th December. I wonder if the name "Apache" was a passing tribute to the American Indian George Mountain Bear whose task was to "keep the prototype clean as possible and waxed in order to pick up those few vital miles per hour" - statement by the prototype test pilot Vance Breese