A c1924 Model 18 Norton; with a few extra bits and pieces fitted that may have to do with the electric lights? Martin: "The ER numbers were first issued by Cambridgeshire in July 1922, and finished in September 1928 when the VE series started." Simon: "Note the usual Best and Lloyd oil pump but the outlet from it does not go to a Y-piece for dripping onto both inlet and exhaust cams - as later - but just goes to a single union above the exhaust cam..."