Collets are used to hold small diameter workpieces and small diameter milling cutters. This is the collet closing nut Taig 3/4-16 (lathe) spindle. It does not fit the newer ER16 headstock. Measure the threaded part of the spindle nose, if it is right at ¾” then you have the ¾-16 headstock. If it is closer to 7/8” then you have the 22MM ER headstock.
Note: DOES NOT FIT THE ER16 HEADSTOCK.