 BSA produced their first four-stroke unit construction singles in 1959 when they introduced the C15 to replace the venerable c12 single.The unit construction (in contrast to the separate engine and gearbox of the C10/C11 and c12) gave the family of motorcycles started by this model its familiar name.The C15 was intended as a utility "get to work" model, and served this purpose faithfully for many thousands of users.