The Colt links were originally supplied to Britain in the late 1930s when the RAF adopted the Colt made export version of the US AN-M2 aircraft gun. The Brits converted the design to .303 feed plate and breechblock and barrel,and they were made by BSA and Vickers initially. After 1939, John Inglis of Toronto also made large numbers of them.
The plated links were typical of Colt’s export production…Colt made several calibers of aircraft Brownings for Western Hemisphere countries, whilst FN supplied European and Asian customers…303 was a popular Air MG Calibre in the 1920s and 30s (Vickers, Lewis, Darne, and Browning and ZB )
What is the Headstamp of the Cartridges??? If it is US CONTRACT …303, then definitely supplied to Britain in WWII.
BRITISH made links are Blued and bear UK makers marks and " Mark"indication
Btw, due to case diameter differences, .303 links are slightly smaller than.30 cal. Links.
Theoretically they are not interchangeable, but a M1919
Link can be crimped( rear and middle rings) and the front ring expanded slightly or left as is to fit and feed .303 ( in a Browning .303 Mark I/II/III gun).