I believe the factory correct finish for all First-Gen A/C receiver/dryers is painted (i.e., dip-primed) semi-gloss black. I ran into this issue back in 2010 when preparing my car for the 2012 Legends judging. In 2012, there were two 68s being judged for Legends that had factory A/C: mine and a coupe by Keith Morsel (not sure of last name spelling). Both of our cars had our receiver dryers painted semi-gloss black and did not receive any point deductions for them. Keith's car took a 100% the following year.
Incidentally, Jeff Ashen ('Pacecar Jeff') of broachbuster.com has climbed in and out of hundreds of First-Gen Camaros in junkyards in his lifetime and can be an authority on originality. His web site sells NOS and rebuilt A/C receiver/dryers:
http://www.broachbuster.com/id49.htm. All of them appear to be painted semi-gloss black.
Bernie