Bofors 40 mm gun

Mark I: twin mounting based on American design and using American built guns, not widely fitted. Fitted for remote fire control .
Mark II: quadruple version similar to Mark I
Mark III: a navalised version of the Army single mounting, hand worked elevation and training.
Mark IV: a tri-axially stabilised twin mounting copied from, and usually known as, the "Hazemeyer". It had on-mounting fire control, and was usually fitted with Radar Type 282 to provide target range information.
Mark V: twin mounting, that superseded and eventually replaced the Mark IV, often referred to as the "utility" mounting. This was a simplified, unstabilised mounting based on the American twin mounting Mark I, and was designed for remote fire control.
Mark VI: a six-barrelled weapon feeding from large trays instead of clips and designed for remote control from a dedicated radar-equipped director.
Mark VII: a single barrelled, hydraulically powered mounting that superseded the Mark III and entered service in 1945.
Mark IX: Mark VII mount modified to electrical power, as the Mounting Mark IX , and in this form saw service in the Falklands War .

The Bofors 40 mm gun is a famous anti-aircraft auto-cannon designed by the Swedish firm of Bofors. It was one of the most popular medium-weight anti-aircraft systems during World War II, used by most of the western Allies as well as various other forces. It is often referred to simply as the Bofors gun.