BSA ROCKET 3 starter motor

[iansoady]

Probably, the versions I'm familiar with are the Mk 3 and Mk 4 analogue systems.

[DMadigan]

Referring to this one:

[iansoady]

But that shows the coils being fed from the BB box? Although I have to say my electronics knowledge is minimal at best.

[DMadigan]

No, not really. The Boyer is actually a negative ground system. If the positive lead to the Boyer were connected to a separate battery or charge pump, then the timing circuit would be separate from the coil driver.
They could have designed it with a PNP coil driver which would put the coils on the negative supply with the driver between the coils and the positive supply. I think at the time there were higher power NPN transistors available than PNP.