A questioner said:
What is this fixture found on the floor of a closet? My home was built in 1937. Any idea?
If you look at the floor, you can make out a roughly 3′ x 1′ rectangle in front of the flange where the varnish is shinier. It wouldn’t be a radiator, they have 2 lines, 1 supply and 1 return. Based on the fact that the outline is centered in the closet and had only 1 line going to it, my guess is that space was created for a gas fireplace or furnace, and that’s where the gas line came in.
Asked my mom (age 73) if she had ever seen anything like that, and she thinks that it may open something in floor. She said to try putting a pin in that tiny hole and rotating it. I don’t know if that really identifies anything but it would be interesting