This is the interrupter wheel for the 7x14 spindle tachometer.
This interrupter wheel can be used to adapt the PN 1684 spindle tachometer to machines with a larger spindle diameter.
The wheel is 66 mm (2.60") in diameter, 1 mm (0.04") thick, and has a 30.2 mm (1.19") hole in it. There is a 5 mm keyway in the bore. The diameter at the root of the teeth is 56 mm (2.20"). There are 40 teeth.
