Three "marks" (I, II, III & III*) of Aircraft Brownings were used by the Commonwealth Air Forces. They were Built by BSA, Birmingham, UK and J.Inglis, Toronto, Canada. The variations refer to the types of Placement and whether there was a Handgrip trigger, a Solenoid or Hydraulic trigger, and Open &/or closed Bolt firing, and the type of Muzzle Cone present.
They were Reversible guns ( double track bolt, for both Left and Right sided Feed,) and the sideplates were the typical Thin Version used on the US ANM2 design. Early in the War, they were used (and Lost) in Large Numbers, as Quadruple Turret Mounts, from Four to Eight per Fighter (Wing Mounts); as Single, hand fired Flexible guns in Flying Boats and Observers Positions, and so on.
They were supplanted by 20mm Cannon in Fighters as the war went on, and by both .50 BMG and 20mm in Bombers by 1944-45.
Their High cyclic rate made them unsuitable to Ground Use, except in an emergency (not enough cooling in still air, and the barrels were very slim and tapered)...There is an account of recovered Guns being used by the Danish resistance in 1944, aided by escaped POW- RAF Gunners...the rule was , short bursts only, well spaced, to prevent jamming due to overheating of the barrel.
Construction wise, typical Browning Rivetted design; Nearly All parts are NOT interchangeable with a M1919 series:
Some of the Big UK (wholesale)traders still have Hundreds of them in stock, in almost new condition.
BTW, the Links for .303 are different from those of .30 cal, but only in the diameter of the circular loops...I have made a "crimping" tool, which brings .30/06 Links down to a good, working Fit, and will cycle through the .303 FeedPlate. I DO have about 5000 original .303 Links in any case...Fruit of about five Years of Milsurp ammo in Aus. set up in Browning Belts ( UK and Canadian .303)
