Not quite the correct answer. Alternators were bare aluminum but with a chromate conversion coating applied. Henkel Alodine 1201 for brush application may be the same or similar. Product description: "Nonflammable, chromic acid-based, coating chemical that will produce a chrome conversion coating on aluminum and its alloys. The coating formed by Alodine 1201 is gold to tan in color and it becomes a part of the aluminum surface."

My information comes from Jim Ridge at Dixie Restoration who does an outstanding job on restoring alternators and starters. According to Jim alternators got the coating.

Photo shows a finish other than bare aluminum with a hint of yellow or tan and a slight rainbow effect.