The easiest way to tell is if you rub your fingers on it and it turns blackish, then it's CAD, if not, it's likely clear/blue zinc. More often then not parts were done in zinc back then. Cad is also very expensive these days and hard to find people to do small lots (or any lots), due to EPA and toxicity. It's much easier to find a zinc plater than a CAD plater...ZINC looks better anyway, IMHO.