It's a baseboard socket in a third floor hallway. There may be more in the house but this is the only one I found. The fact that it's in a hallway may be why it never got changed to a regular outlet. There are also the four prong duplex outlets in there, so I'm guessing upgrades were done throughout the history of the property.
If I recall correctly, the first generation appliances like radios, irons, table lamps etc. came with a cord end that screwed into these sockets. Plug-in cords weren't invented till later. That's why they used to make the three socket adapters with a pull chain on the one coming straight down. The other two were for appliances.
This just reminded me that when I started back in the 80s doing resi work, the older generation used to refer to receptacles as sockets. I think this is why.