Alternator Question?

I can think of one reason for that ground screw. The guy at a part store who will mount it on a tester to spin it up will have a ground wire that needs to be put somewhere. A clearly marked ground stud removes confusion, keeps the clip off of fins and away from the inside components.
Maybe the factories engineers finally determined that relying on painted brackets and hardware for a chassis ground path wasn't the best plan/could be improved. Took long enough huh ? LOL
Whether there are 2 brush holders there or not, I'll back away, hide and watch. We don't know more than "Dart" about this application anyway.