Back up here a minute - have you got a 70? If so you need teh alternator you have with the 2 brushes. It has had nothing more done the way I read this than the 'pig tail' to the one brush grounded instead of going spicing into the keyed wire. Extend this wire and got where it belongs and use the proper regulator. Put in brushes if you feel they are needed. Not a very hard job and cheap to do. That is provided that the other wiresgoes to a regulator. do you have the electronic regulator as with the terminal as pictured. A 70 should have this one.
it sounds to me like some one grounded the one brush in order to use the alternator on a single brush system as in 69 and down using the small rectangle box regulator.