Alternator charging Question

This is easy. All (what you are calling dual field) "isolated field" alternators have a winding for the field, each end is hooked to an insulated brush.

THERE SHOULD BE NO continuity to ground from either brush holder

There are about three possibilities:

ONE A chicken-sh$$ rebuilder grounded one of the brushes to allow it to be used in a 69/ earlier car

TWO One of the brush holders is wrong, damaged, or missing a piece and is shorting to ground

THREE The winding itself is shorted to the frame of the rotor.

Easy to find out. Use your test lamp and a battery, or an ohmeter

Neither brush should have continuity to ground. Try removing one brush at a time (and examine them) and see if the short is removed.