The Vickers-Berthier was initially designed by Andre Berthier in France prior to World War One. It went through a number of substantial design changes before the war, and was actually ordered in quantity by the United States right at the end of WWI – but the order was cancelled with the armistice. In the 1920s, Berthier sold the design to the Vickers company in England, which wanted a light machine gun to market alongside its Vickers heavy machine gun.
When the British military decided to replace its Lewis and Hotchkiss light machine guns, the Vickers-Berthier was one of the leading contenders, although in the endurance trials it was edged out by the Czech ZB-33, which would ultimately be adopted as the Bren. However, the Indian Army opted to take the Vickers-Berthier, and it was put into production at the Ishapore Rifle Factory and saw substantial use in World War Two.
Mechanically, the Vickers-Berthier is a tilting bolt design with a long stroke gas piston. It has a thorough set of covers over the magazine well and ejection port, and a relatively slow rate of fire. The barrel is quick-changeable, and it feeds from top-mounted 30-round magazines, with an aperture type rear sight being offset to the left side of the gun to clear the magazine.
Thanks to Marstar for letting me examine and shoot their Vickers-Berthier!