Could've been a vendor thing, too. Normally, a booster and master cylinder are not painted as an assembly. So, let's say you have a black or cad-plated booster, your master cylinder is going to be as-cast (not plated or painted), and the covers I can ever remember seeing have been either unpainted or cad-plated. I agree that a person would need to research original factory brochures to get a better idea on what is right for a particular model and year.