1973 Scamp won't start in start only in run. With a Twist....

If your voltage split is 10.5 and 6
Then either the battery is in very poor shape, or the juice is just not getting to the coil, and 6volts may not be enough to power up the ECU. I know it runs down to 9.5v, but never tried 6

With NO BALLAST, the coil should be seeing within 1/2 volt of battery volts in either position.
If the starter is drawing the battery down, such that only 6v is seen at the coil during cranking.......Something is wrong! A good battery will maintain 10.5 volts during cranking, and therefore I would expect 10V at the coil, during cranking.
Either the battery is NDG, the starter is shorted, or the engine is very difficult to crank.

OOps post #1 says you have a dual ballast