This rifle refers to a British Short Magazine Lee-Enfield rifle (SMLE), specifically the No. 1 Mk III variant, produced by the Birmingham Small Arms Company (BSA) in 1908. The "ER" likely signifies "Edward Rex," referring to King Edward VII, who was the monarch at the time of its adoption. "BSAC" indicates it was manufactured by BSA, and "ShtLE III" denotes the Short, Magazine, Lee-Enfield, Mark III model. These rifles were widely used by British and Commonwealth forces in World War I and beyond