Gearbox sprocket, choose your number of teeth: 18, 19, 20 or 21 teeth.
Chain: 5/8 x 3/8 | 530 | 110056
BSA A7, A10 fits both: the ealy bolt-on gearbox (plunger frame) and STD gearbox (swing arm)
BSA B31, B33, B34 with STD gearbox.
Part numbers: 67-3064 /18T, 67-3065 /19T, 67-1111 /20T, 67-3065 /21