5/8" diameter rubber head nails/cabinet bumpers for use on piano cabinets.  Actually about 1/64" under 5/8" diameter - making a true measurement of 39/64".  Head is about 5/16" tall. Most often used to protect cabinet surfaces that would otherwise touch and mar each other. These are pretty large and I more often encounter the smaller sizes for piano applications (though these are also used for other items).  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 brown in my eBay store, also available in 2 smaller sizes (1/2" and 3/8") in brown and black.