Gearbox front sprocket, choose your number of teeth: 18 or 19 teeth.
For the narrower type rear chain: 5/8 x 1/4 | 520 | 110-054
BSA A7, A10 fits both: the early bolt-on gearbox (plunger frame) and STD gearbox (swing arm)
BSA B31, B33, B34 with STD gearbox.
Part numbers: 42-3016 /18T & 42-3016 /19T