5/8" diameter** rubber head nails/cabinet bumpers for use on piano cabinets.  Rubber head is about 5/16" tall.  Most often used to protect cabinet surfaces that would otherwise touch and mar each other. Frankly these are pretty large and I encounter more of the smaller sizes.  Bumpers are usually found on the keyslip (strip of wood across front of piano just under the key fronts), fallboard/key cover, occasionally the cheekblocks, under bench lids and piano top lids.  Also available in black in my eBay store, also available in 2 smaller sizes.

These are apparently also used as feet on antique/vintage radios.

**It has been brought to my attention that these actually measure about 1/64" under 5/8"**

I've been a tech 30+ years and am glad to help.  If you have questions about this item before or after the sale please let me know, thanks!