Thanks very much Rick. I believe that the dynamo is a C35C which has two brushes. It was refurbished a few years ago and seems to work well. I shall digest the article to which you referr in the next day or two, but as my knowledge of electrics in general and car electrics in particular is very poor, I will have to read it VERY SLOWLY!
The Ruby diagram is very close to the RP and the inner workings of the rotary switch are made apparent. If the original switch has been replaced by a simple version without the Summer facility then there are posts covering that.
A 2 brush dynamo indicates mods, with all the confusion which goes with these if not documented with the car. Car may or may not retain a two stage facility, or may have acquired a regulator of some sort etc, all of which can make it difficult for amateurs.