During the 1930s BSA concentrated on producing a range of dependable, well-made, competitively priced motorcycles. When BSA's single-cylinder range was redesigned by Val Page for 1937, the engines took on many of the characteristics they would retain in the post-war era, most notably dry-sump lubrication and gear drive to a rear-mounted magneto. There were three new offerings in the 250 class: the sidevalve B20 and overhead-valve B21 and Empire Star models, the latter featuring foot change as standard on the four-speed gearbox shared with the others. Production of the popular B-Series 250s continued to the decade's end when the range was superseded by the new C-Series quarter-litre models: the C10 and C11. Last taxed in 1961, this incomplete B21 is offered for restoration and comes with a V5C document. It appears to have been acquired for the collection in 1993.