Just FYI: My 67 C20 has an alternator and an external regulator. You can tell them apart but it can be a pain in the #$%. Look for insulators on screws. They hold the regulator in place. Not on the posts that the wire goes to. Some alternators have enough vents in the back you can see the circuit board in there. And some do not. Part number is the best way and all of these ideas require that you pull it off and look at it. No short cuts that will keep you warm and dry come to mind.

Steve H