I call them "unfindables". After oh, say ten years or so when I wish to remove a sight/scope base from a firearm - this are the things I need to find to fill the holes. I can never find them. When I run across a small plastic sack (such as you posted), the sack was never labelled.
Basic rule is - the ones I find will not fit the holes I try to fill. Based on prices asked on Gunbroker/FleaBay, a thimble full is worth a tank of gasoline. Many guns have pre-drilled holes in receivers for mounting sight or scope bases. Save them.