That would be the 280 Westley Richards I believe. The rim fits a standard Mauser bolt face, which my rifle has. https://www.cartridgecollector.net/280-westley-richards-rebated-rim
The question is, why would Lancaster chamber a rifle in a competitors cartridge? Lancaster was Ross' agent on London. They were personal friends. To quote from Terry Wieland's book "Great Hunting Rifles, Victorian to Present":
“Although the .280 Ross was initially a target cartridge, its designer intended it also as a military and hunting round. After the 1908 Bisley triumph, gunmakers began chambering hunting rifles for it, as well as developing proprietary cartridges to compete with it. In Germany, Mauser’s new magnum action suited the .280 Ross perfectly, and it became a standard Mauser chambering. Ross’s friends at Charles Lancaster & Co. chambered the .280 Ross in their bolt rifles, and also developed a flanged (rimmed) version for use in double rifles and single-shots”.