This is a truss rod adjustment screwdriver from StewMac, the most popular brand for guitar maintenance tools. A flathead screwdriver can be used as a substitute, but this is better for guitars you want to take good care of, as it won't gouge the wood. It's for Fender guitars, but similar ones can be used for other brands. It's been used only once and is in excellent condition. Due to the current weak yen, a new one costs 3000 yen plus shipping, so it's a bargain if you don't mind a used one.