This thing is right in the center of my bedroom’s floor, which is a converted dining room in a 1920s apartment. I can’t figure out what it is.
Answer: This is a servant floor call button.
My mother told me about seeing these at a relative’s house when she was very little. She said she found it entertaining, during a dinner party, to slip her foot under the table and press the button, and then see the hostess get furious at the servants for bringing in the next dish before the guests were even ready.