The three wired plug on the early alts went to the voltage regulator. The complete brush holder can be simply replaced with the common cheap Bosch internal regulator used after 1975 (I think). Part number 30 060 (I think).
By doing this one has eliminated the harness to the old regulator (and the need for the regulator). At that point there are two wires and maybe a ground. I would look closely at the ground. The alt is rubber mounted and needs a separate wire. Leaving the ground open could take out an alternator rather quickly.
Bosch Master, ASE Master, L1
33 years MB technician