Several people that I've asked have said a tub overfill drain cover and I don't think they'd make that out of cast iron you little rusty I think I know what it is but I don't know the name of it so if anyone knows what this thing is called I would be forever indebted
I notice that the two mounting holes are elongated meaning that it was adjustable ? I would say it was an eye for a lever, hook, adjuster, etc for a piece of machinery ?
When you say tubs I assume you mean bath tubs. The early tubs didn't have drain levers or such. All they had were rubber plugs or what we called stoppers.