Under the steam radiators.

Is that top part cast iron?

I'd say they were to spread the radiator's weight around so the legs wouldn't wear out the floor.

probably help them slide as they expand and contract too

Any chance there is a patent number on them somewhere? They look like someone's idea that never caught on.

I have never seen them before. I have a few guesses. 1) they are not radiator parts, but are designed to work with a machine,? or slip over a bolt or bracket to be removed easily, as a spacer during some manufacturing process?

All radiators! sure. I spent a life of repurposing materials and parts from everything. I always considered it a bonus when I had lots of the same parts to use elsewhere. there was a guy who insulated his house with movie theater posters. He ran a movie theater. the posters were cut exactly the width of his 2*4 wall frame. when it was discovered years later, the posters being so rare, became a money windfall for the new owner. The art world is full of examples of materials turned into art. The only thing that made me think these brackets are not radiator leg supports is the open end and its uniform space as if it was a tool designed to wear that slipped over a bolt or rod. Like a spring tensioned bracket, the raised nodules on the one side lock it into place. Still, I do not know where they came from. I tried image search and radiator catalog search. It will probably be the previous owner, or a best friend machinist operator who would ever know. I saw a tool and die factory close that scrapped tons of stuff with large quantities of same parts. no longer even made or used.

Lost to history I say. It would be ironic to think it came from a foundry making radiators, but only from a machine used there.🙃 The internet is big, but reality is bigger and older.