The original comment:
" The Mustang was designed to a British specification for procurement by the RAF. To me that makes it a British aircraft which was produced in an American factory."
is not "pretty much correct". It is also the opposite of what you said yourself. Even if the USAAF had never adopted the Mustang, it would not be a British aircraft unless it was also manufactured entirely in Britain, any more than the P-63 was a Soviet fighter. This statement was one of those deliberate statements that take a grain of truth (the British asked NA Aviation to produce fighters for them) and exaggerate it all out of proportion to make make an interesting claim. I'm not criticisng here - we all do that.
Actually, it was my understanding that the RAF only asked NA to produce P-40s for them - it was NA itself that proposed designing and building their own fighter. The Mustang is 100% American
It was still from a British design requirement.
Again I am not saying it was a British Aircraft, and have never said that.