The C stands for 'Charge'. From your description this is an older type switch and somewhat rare, so don't give it away to anyone in exchange for a packet of crisps! I've seen it on tank panel bike of the 1930s. My 1931 Matchless has one.
I don't know if you can place it directly on an M20. You'd need to consult your wiring diagram for that. Have a look at the underside. Some of this type of switch have a built in resistor.
As Allan said. the "C" is for charge. It was for the old 3 brush dynamos that only had a cut out and weren't regulated properly. From memory, I think you switched it to charge if you weren't using lights, to reduce the charge going to your battery. Maybe someone else can confirm? Ron
Yes, Ron is right (I've never known him not to be!). This switch is indeed for the three brush dynamos with the cut-out. The rider was able to switch to 'charge' when on long and steady trips and top up the battery. When the lights were on either H or L the battery still could charge, but only above certain revs, which is why the ammeter was useful. You could look down and see if you were using more power than you were generating. The Off position, of course, switched off all charging.
The wm20 doesn't need the charge feature as that is handled by the control box, if I understand things correctly. The technical section of this site will give you some excellent guidance about switches.
