It is belt driven off the accessory case. One possible explanation was a
sticking crankshaft counterweight. However, the engine has not shown any
indications of vibrations, such as smoking rivets, cracked baffling, etc.

However, until the new belt was installed, there had never been any problems
with the electrical systems.

I suspect, but will never be able to prove, was that the belt was either not
tightened enough when changed, or the field wire was knocked loose or
kinked, causing the alternator to run intermittently. On inspection after
the fact today, it was corroded.

My standing there watching would not have made any difference- there would
have been no way to tell if the wire was broken, and short of taking the
wrench and tightening the alternator myself there was no way I could tell if
it was loose.

I ran the plane for nearly an hour after the work today, with none of the
same indications, including under a heavy electrical load with lights, etc
in relatively high IMC.

Hopefully, it'll be back to where it was prior to the inspection.