The North American Mustang, in R.A.F. service it was known as the Mustang Mk I. With the first prototype delivered an astonishing 149 days as the NA-73X after the first contact, with the first delivery of 320 NA-73’s (It wasn’t the P-51 yet!) following on with 300 NA-83’s all designated the Mustang Mk I by the R.A.F.
The evolution of the aircraft is well understood, all the way to the P-51K in R.A.F. service the Mustang Mk IVa. Many of the Mustangs were assembled at Renfrew at the Lockheed facility, shipped from there to a number of different units.
With the exception of the earliest orders for the Mustang Mk I, which were struck off charge in 1945. All the rest of the Mustangs in R.A.F. service were lend lease and so had to be returned to the United States or scrapped. The last Mustangs in R.A.F service went in 1947, so the Mustang in the picture in R.A.F. colours is actually a rareity – there aren’t many Mustang Mk IV’s around.
There is some excellent information in the book, Lend Lease Aircraft in WWII – by Arthur Pearcy. The book which is old now and out of print is a must if you are researching aircraft during the period.