A 1936 Scott was equipped with a Lucas Magdyno with no voltage regulator. It was a third-brush dynamo regulated by the battery – if the battery was not in circuit, the voltage would easily increase to more than 30 Volts. To regulate the dynamo output in Watts, there was a small spiral-wound resistor mounted inside the headlamp which normally damped the amount of current reaching the dynamo field windings. When the lights were switched on, the resistor was no longer active, and the current to the field circuit increased, thereby increasing the dynamo output.
As far as I remember, A Lucas Magdyno with an outside boxed regulator was first used on Scotts in 1938. A Bosch unit cannot be original and must be a later addition.
Hi Carl your date of 1938 is a bit late, I have got a copy of the road test from The Motor Cycle for March 11 1937 in front of me now , the actual words are neatlymounted below the saddle is the voltage control unit, a new feature. it also shows it on another picture on may 11 1937. Eric