If the electrical contractor who helped me with the wiring for my exterior RV outlets at my home is correct, these outlets are obsolete and the only hope of obtaining either the outlet or the plate/cover assembly is to find an NOS unit through an electrical parts house. The one such outlet that I have on my '64 Overlander had a metal cover that polished very nicely - - the screw-on cover on the other hand is a cast aluminum and did not polish well. The photo below (taken on a gray overcast rainy day is not of the best quality) illustrates the difference in sheen between the cover plate and the cap.
The receptacle on my coach is for the factory optional porch light that can be seen in the photo below:
I have been on the lookout for these receptacles, but haven't run across any - - did happen to find an electrical supply house where the counterperson remembered selling the last one that they had about a year ago.