If your Cashmore is a 1941 gun, then it will have been proofed under the 1925 rules of proof. That means the chamber length should be stamped on the barrel flats, along with other proofmarks. A 1941 gun would most likely have been made with 2.5" chambers, although 2 3/4" certainly isn't impossible. But even if it has 2 3/4" chambers, that doesn't mean you should shoot American factory 2 3/4" loads in the gun. US (SAAMI) service pressure is much higher than British service pressure.