I didn't want to flip my 6500 upside down to take a pic, but the feet are round and rather small - perhaps 10 mm in diameter and only about 2 mm in height. They are very near the corners of the bottom plate and probably snap into round holes.
They seem like hard plastic rather than rubber, or perhaps they just turn that way in 20 years...
I think I would just try the kind of adhesive felt pads that go on the feet of furniture. The thickness is about right and they're not going to mark the surface under them, like some kinds of rubber do.