Coils and ballast resistors

I am no expert but for vehicles with electric start it was a method of keeping the circuit voltage within spec when you had the cranking load on reducing the voltage.  When the motor was running the ballast resistor would deal with the excess voltage.

J

it was down to size .2 12v coils hard to hide  6v coils half the size 

Seems no one knows or can speculate.

The ballast/6v coils were engineered into commando for the planned 71 20M3S (starter) which was intended to receive the new starter configuration. Since it was a disaster, the starter was not used but the new ballast/6v coil & wiring change had been allowed to remain. A ballast bypass  would normally be used during starter cranking. Ballast bypass is obviously used on MK3 E-Start.

The reply above re size of 6V coils and 12V coils is rubbish. We have always had large 52mm and 40mm coils since the middle 60's.   The real reason for 6V coils and ballast resistors is with the electric start when the starter pulls the battery voltage down, the starter solenoid removes the ballast resistor so allowing the 6V coils to get 10V and stand a chance of starting. If you had 12V coils at 10V far less chance of starting. David Comeau answer above is correct. The system is used on cars for many years. Bear in mind once you go electronic with your ignition system then this system is totally lost, but it is very useful to have your two 6v coils in series for wasted spark. Going back to the original post. If you have points and a kick start then there is no advantage for the 6V coils +ballast over 12V. Except that the ballast is yet another electrical item to fail (Modestly unreliable item anyway) so 12V coils would be the way to go.