Try a magnet. If it sticks, they're plated. If it doesn't, likely stainless or another alloy. Some SS alloys will attract a magnet, so this test is not necessarily definitive. Alternately, you could pry a floor glide glide from the bottom to see if there's surface rust on the inside.
@khus, Appears to be a brushed finish and, to complicate things perhaps unnecessarily, possibly lacquer over bare mild steel.
Is there evidence of rust on the top of the mounting flange?
Either way, I'd probably just use #0000 steel wool with a slurry of baking soda/water and rub gently parallel to the direction of the brushed finish, rinse and dry thoroughly, and maybe respray with compatible lacquer. Don't go too crazy. A little pitting and/or discoloration is considered normal, honest patina on vintage pieces.
Thanks tkt, good ideas. Yes, it is a brushed satin finish. There is no rust on the top side of the flange as I suppose it has been protected against the wood, but the bottom side of the flange does have some. I agree, a little patina is ok. Mostly want to shore them up to prevent further decay. However, I do have two out of eight that are a bit more troublesome in appearance. Will see if I can bring them back or check into re-plating. I have had many items re-chromed, but always with a mirror finish. Just checked the website of the place I have used and it appears they do a satin finish. I do have a reproduction Herman Miller catalog from 1952. While they give good detail on the wood finishes and pulls, when they list the legs they just say, "metal."
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