high voltage stock 6.1 alternator

They adjust the voltage sensing with that potentiometer.
A wild guess would be, that this potentiometer could adjust the voltage "seen" by the VR,
basically "on"-time of the field excitation winding.

14.8V is not too far off a good charging voltage. You are shooting for 14.2 - 14.4 but i would say that 14.8 is acceptable.