A precision micrometer rear sight assembly produced by the Birmingham Small Arms Company (BSA), designed for use on BSA Martini-actioned smallbore training rifles chambered in .22LR, Morris tube conversions, or Rook Rifles. The sight features finely knurled adjustment drums for windage and elevation, allowing for highly accurate zeroing. The curved base plate is typical for mounting onto the rounded receiver of the small-frame Martini actions. Such sights were widely used throughout the British Empire and Commonwealth during the late Victorian, Edwardian, and interwar periods for military marksmanship training and civilian target shooting. The Morris conversion system allowed full-size military rifles to be used as training platforms with .22 rimfire ammunition, while purpose-built BSA Martini .22LR rifles became mainstays in cadet training and club competitions. The BSA sight you show is a highly desirable original, fully adjustable, and correct for many late 19th and early 20th century falling block rifles.