The D1 electrics have been the cause of some concern with the tendency to 'blow' bulbs. I've removed the stator plate, to reveal the coils (see picture), as I wanted to see what was behind the plate.
Here we can see it's a three-coil system (ignition coil removed, by the way): two large lighting coils with the red wire, and a smaller charging coil with the green wire. The white wire is the earth wire, common to both lighting and charging.
Strangely enough, only the red wire was being used in the set up, with a black wire joining it and going to the lighting switch. It seems, from what I've gathered, the charging coil was included by BSA if the owner ever wished to convert from AC to DC! (Can that really be so?)
Meanwhile, my friend Hughie is trying to sort out the stop plate mounting lugs! A bit of grinding, then head-scratching, a bit of welding, then heating it all up... what will happen next, I wonder?