The Mustang MkIII (P51B/C) had the reflector gunsight, (MkII I believe?) and, as stated by 'dog above, the Mustang MkIV (P51D/K) also had the reflector sight in R.A.F. service generally. A friend of mine flew MkIV's in 1945, and his A/C had a reflector sight.
One of the possible reasons that Mustang III's did not receive the upgrade, apart from the fact that stocks of GSCGS's seemed to be reserved for Spitfires, is that the refelector sight on MkIII's was mounted through the instrument panel shroud, and the Gyro Stabilised sight, plus its mounting, would not conveniently fit. Or so my friend told me! But yes, the British GSCGS started to be itroduced in late 1943 to Spitfire squadrons, and design data was passed to America to aid in design/production of the K14.
Hope this helps.