I'm renovating a Shizuoka ST-N CNC knee mill with a Bandit 3 controller from ~1980 in it. The spindle drive is a Todai Electric Co DC Motor Control Pack model number MOP-1500 which has absolutely no documentation online.
There are two Fuji Electric SRC3631-5-1 magnetic contactors in it, one for each spindle direction. However, one of them has this strange add-on component on the bottom of it.
The picture below is from an ebay listing with an identical add-on, except theirs has a knob with labels 4, 6, 8, ARC, while mine just has a large plastic phillips screw in the same position.
The small white rectangle next to the knob is a piece of loose plastic in a slot, and below it there's a button which it can press. In the configuration I received this part in, that button is held down by some sort of internal toggle mechanism, so pushing or pulling the white plastic does nothing. However, if I turn the knob a quarter turn clockwise, the toggle releases and the button becomes a normally open momentary switch which I can close by pushing on the white plastic. If I turn the knob back, the switch stays open until I press it again, and then the toggle engages to keep the circuit closed.
What is this add-on called? What's its point?
My best guess is that it's some sort of safety feature, where if something passes a threshold (adjustable with the knob) the switch will momentarily open. But, I need to know what it is because perhaps the toggle is the safety feature, and the operational state is un-toggled, with the circuit open.