Here are two unusual piano insulators -- Note these are much larger and thicker than any casters -- take the pair for $20 plus actual postage

large round piano insulator -- 4 1/8" diameter -- cool cross hatched bottom with FMJ around a star (I believe this is a French maker) -- condition is really nice with just a little wear in the cup

A little smaller round piano insulator -- 3 3/4" diameter -- cross hatched base with no markings -- small base edge bruise and a couple tiny flakes