This is the interrupter wheel for the PN 1684 spindle tachometer when it is used on a 7x10 or 7x12 mini lathe.
The wheel is 56 mm (2.20") in diameter, 1 mm (0.04") thick, and has a 27.2 mm (1.07") hole in it. The diameter at the root of the teeth is 46 mm (1.81"). There are 40 teeth.
