3/8" diameter rubber head nails/cabinet bumpers for use on piano cabinets.  Head is about 1/4" tall.  Most often used to protect cabinet surfaces that would otherwise touch and mar each other.  Found often on the keyslip (strip of wood across front of piano just under the key fronts), fallboard/key cover and sometimes the cheekblocks.  Also used under bench lids and piano top lids (though those are sometimes the next size up - 1/2").  Also available in brown and off white in my eBay store, also available in 2 larger sizes.