By 1958 BSA's Bantam was entering its tenth year of production. Developments in that time had included a 150cc version, which also had a swinging arm frame. Perhaps to help convince the public that changes went deeper than revised colour schemes, the manufacturer decided the model number should now reflect the power output. Thus, the 175cc model that was new for 1958 became the D7. The numbers increased in 1966 when the D10 arrived. Combined with a four-speed gearbox, the additonal urge could be used more effectively.
Work continued and various changes resulted in the liberation of yet more horsepower. Production of the new models started in 1968 and, with 12.6bhp available, the system decreed the new model was the D13. The first 680 bikes featured frame and engine numbers in this series, but mindful of sales to the superstitious, BSA tweaked the number and the model went forward, selling thousands more, as the D14.
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Thus, the D13 may well be the rarest Bantam of them all. In spite of that, the BSA Bantam Club managed to find two examples as part of the club's display at the Amberley Motorcycle Show in May.