Saw your post over in another thread. I've never seen one like that either in a Regal. I think it's RPO code C89. Looks like it uses parts from the optional 1/4 lamps used in the 79-85 Rivieras. And I don't seem to find any part numbers for it in any of the parts catalogs and no information or pictures in the factory shop manuals either.
BTW that car also has ConcertSound2 in it. Did you snag the rare lower door trim panels? And those Opera Lamps?
I do sure miss going to the LKQ yard in VB.