I agree with Uwe -- if that outlet is mounted higher up on the side of the coach it was likely intended for an exterior lamp -- my '64 Overlander has had one since it was delivered in 1964
-- a photo appears below:
It is very difficult to find the original style cover, and mine was restored as much as possible by the Ruth's when they polished and Plasticoated the exterior. The fixture itself is rather difficult to find, but was one utilized by manufacturers other than Airstream as my family's '69 SunWay had a virtually identical fixture. Through a stroke of luck, I was able to find an NOS glass shade at one of the Annual WBCCI International Rally Flea Markets.
Good luck with your search for a replacement receptacle.