The top piece reminds me of a boot scraper out on the farms useful to not get animal poop in the house.
I’d call it a plant stand as well.
It looks like an old register cover that has been made into a plant stand.
Old heater vent cover? Plant stand sounds perfect.
It's a 19th century brass trivet on an iron base, likely used in a large fireplace to set a tea kettle on to heat the water.
Yep the trivet answer is the winner
Old Brass & Iron Trivet that was used for kettles in the fireplace [ to keep the liquid hot ] . Saw many just like yours when I searched via Google Images - fireplace cast iron trivet
and then visited one site's image...
I would use it for a plant stand or let it hold a tea kettle
Definitely not modified in any way, because I have one that looks just like this one. Think Car19575601 has the closest answer. It's on the small side so it would only hold a small plant.