Actually, it was a break in the negative terminal battery cable. The break was inside the insulation. The ground connection is otherwise solid. It came apart when i was roughening up the surface a bit with a brush. This yet another reminder to me from an inanimate object: Be more thorough!

No surprise to you all, then
The battery is still strong. the alternator produces a charging current. I'll still take a look at the bushes and regulator tomorrow. The alternator is (my guess here) 6 years old.

Thanks incredibly for the tips (and not knocking me . Forums like this are absolutely indispensable.