A Course setting Mk IX C Bomb Sight, Die casters Ltd, Melbourne, circa 1930s, incised 'R.A.A.F Ident. No. E9/494, Course setting Bomb Sight Mk IX C, Serial no. Cac/1514, manufactured for Commonwealth Aircraft Corp Pty Ltd. By Die casters Ltd, Melbourne, Australia, mounted on wooden base, accompanied by original S.G. Brown Ltd type F headphones, the wooden base 56.5 x 12.5 cm. Catalogue note: first invented in 1917, the Course setting Bomb Sight (Csbs) was the first practical system for properly accounting for the effects of wind during the dropping of bombs