I think the air cleaner should be all semigloss black

I am assuming you are referring to a 61 Impala? From GM examples I have seen and numerous cars I have dismantled, areas of the body which were not painted with the color the vehicle left the factory in were primer colored and not primer gray. The problem that one may encounter is different plants may have used different procedures or in some rare cases, different colored primers. It is possible that the color was a flat black. Does it appear blended in to the regular paint color or in areas where a paint coat could penetrate?