Persons before me have replaced the alternator and regulator. (a
note here,
the regulator is the simple 3 wire unit)
All grounds were cleaned and remade.

It would be interesting to know why the alternator and regulator
were replaced in the first place. Perhaps the same problem? I'd check
that old pull-type master switch to see if the alternator control side
of it is dirty and letting the alternator relay fall open or chatter.
Those old switches had grease in them that got hard with age and
collected dust.

Dan