After the conclusion of World War One, the RAF was looking for a replacement for the Lewis for an observer’s gun in aircraft use. The Lewis had reliability issues, and was not a particularly fast-firing gun, and a high rate of fire was essential for effective aircraft use. In response, the Vickers company developed the Class E and F guns for RAF testing and commercial sale. Both were air-cooled, and the Class E was belt fed while the Class F used interchangeable feed block to feed from either belts or 97-round aircraft Lewis gun drums.
These variants were both mechanically very similar to the British service Vickers gun, but with changes made mostly to reduce weight. The barrel jackets were reduced to about an inch in diameter, the trigger was changed to a single pistol grip, the fusee spring cover was removed, and the sideplates lightened. The Class F was tested by the RAF and a few guns put in military service, but it was never officially adopted by England. More than a few other countries purchased the guns, though, including Argentina, Bolivia, Brazil, Chile, China, Finland, Japan, Lithuania, the Netherlands, Peru, Siam, and Spain.
We have a copy of the Vickers commercial manual for the gun available for download:
Vickers Class F Commercial Manual (English)