I have seen a lot of strange outlets in my day but its a first for this one. 1930’s - 1940’s house originally wired with K&T.
That’s an old Aerial jack for a roof antenna. Here’s a photo of one that I removed 10 coats of paint from.
Good find Robert. Talk about mixing low volt and line volt. Its funny on both my picture and your picture you can see all the scratches from people trying to insert a 2 prong in to it.
Since the device in my photo had been there for about 60 years I’m sure that quite a few people tried to plug something in to the Aerial portion of the receptacle. I found this while doing a renovation on a house built in the 40’s. Figured it was worth keeping since I hadn’t seen one either. I removed the paint with paint remover and the lettering appeared.