Wurger is quite correct, the P51B/C Mustang used the gunsights illustrated, as did the P51D Mustang, until late 1944, and often beyond. Note that this sight was normally mounted so that it protruded through the instrument cowl on the P51B/C.
The K14 gyro stabilised computing sight was a licenced version, with modifications, of the British designed MkII gyro sight, and was introduced into the USAAF in Europe from around August/September 1944.