per what I have read I think is good to let you know for those don't know:

The need for ground one of the dual prongs alts to be matched to earliers charging systems is because both are isolated. Rotor needs to get both fields to be able to produce electricity.

On earliers systems ( mech reg ) with one wire for field, this wire ( green ) provides a positive regulated source while the other brush is attached directly to chassis. this closes the field circuit to rotor.

on dual prongs with two wires ( elect reg ), one wire ( blue ) provides constant positive from ign switch, while the other one ( green ) is now a negative regulated source. Hence the reason to be both isolated.

There is somehow of myth about "one wire alts" being more efficient... false. is not because is a "one wire alt" but because the alt itself response better or not to the regulator or regulator is more efficient.

stock look "one wire alts" are not that the same alt with a MOUNTED ON regulator. any of us can become the stock alt into a One wire alt just getting this regulator:



which is available at several dealers. this uses just a ground source, positive source and is triggered to ON from a stator lead