A background plate to make it easier to see the hook fixed to the vise.Please use it by attaching to the shaft of the vise.You can choose either dark gray or white plate, whichever color makes it easier to see the tying fly.It also functions as a connector to fix the parachute tool (sold separately). Dark gray and white Double face plate(Dark Gray and White) Eye fatigue can be reduced by selecting the color that is easier to see.The Parachute Tool available separately requires a sight plate connector for installation.*The diameter of the mounting part is 9.6mm.(0.37inch)