The modification for the external adjustment was done by a local workshop specialized in starters and alternators. They've been in the business for as long as I can remember and know their stuff. We are are talking about a customized job solved with a simple approach. He used a standard voltage regulator, the ones that have the regulator and brushes as one unit and just popped off the regulator. Then he cut a piece of cable with 3 wires and soldered on the brushholder contacts and at the regulator itself. Then some epoxy was applied to cover the rear of the regulator. Bosch do not make regulators with an adjustment potmeter, but other manufacturers do.
The pictures will speak for themselves so you get the idea. One could of course drop the cable all together, but I liked the idea of adjusting it if I felt like experimenting a litlle bit (which is a tendency of mine) Keeping it higher during the cold season f. ex.
On the last picture you can se that the regulator is just hidden under the crossframe above the headlamp.
My battery is from a scandinavian chain called Biltema. I attached a picture of that as well.
