On the eve of World War One, the Royal Horse Artillery and Royal Flying Corps had both adopted the Webley Self-Loading Pistol, chambered for a special .455 caliber semi-rimmed cartridge. With the needs of wartime, Webley deliveries of these pistols were too slow, and alternatives were sought. The obvious choice was the Colt 1911 pistol, already in service elsewhere with British troops. Colt had no trouble adapting the design to the .455 Webley semiauto cartridge, and between 1915 and 1919 some 13,000 of them were sold to Britain (plus more in .45 ACP). The .455 caliber guns are so marked on the slide, and all have W-prefix serial numbers to indicate their chambering. Purchases continued after the Royal Flying Corps was renamed the Royal Air Force in April 1918. While some were surplussed after the war, others would stay in service until World War Two.