A replacement screw for the clamping caps on a locking nut, the sort fitted to Floyd Rose style tremolo guitars. Black socket-head cap screw with an Allen drive and a metric thread, which is what the locking nuts on Chinese and Japanese built guitars use, so check yours is not an imperial US nut before ordering. Worth keeping a spare in the case, because these are the screws that end up on the floor mid string change. AMS part number KP22.