[BomberBoy]

I got a repklly from Martin Kyburz, of the site Swiss Mustangs (http://www.swissmustangs.ch):

Mustangs were delivered to the RAF without gunsights - the RAF installed their own upon arrival - most common was the Mk. II Reflector Gunsight as seen above, but some also got the Mk. IIc and later Mk. IV Gyro Gunsight. The latter were also know as "Ace Maker" gunsights - variants were produced in the U.S. as USAAF K-14 and US Navy MK18

So, I suppose a Mustang III mod with a K-14 sight would in fact be historically correct!