Obviously developped from the Britsh Barr&Strout G.M.2 (designated Mark II in the R.A.F.). The range and span dials found in the British gunsight have been eliminated. The bulb holder at the base of the sight is also different and interesting as it held two bulbs: if the bulb in use burned, you just had to rotate the bulb holder 180 degrees and the spare bulb was immediately on. Much more practical and faster than the British design. Used on the Boomerang and probably other CAC aircrafts.