That alternator would be a 70 right. 8th week. Late 70 build.

There are restorations of cast aluminum that are glass beaded with the proper bead, cleaned, tumbled to get a more correct finish. Then can be coated with a conversion.

Frankly, if an alternator or any aluminum cast part has had oxidation and corrosion the surface has been changed. Let alone absorption of contaminants. Now you can't sand blast it and remove metal. Beading is about removing contaminants and closing pores (peening).

If you see enough alternators you will also notice a lot of mismatched parts, and those industrial tumblers used by the rebuilder would remove a lot of metal, including casting markings and the stampings. Let alone they drilled extra holes for additional field wire.

So any process can change the surface if not done properly.