Looks like an own design, presumably by Denford.

It looks as if the circular PCB has reflective pads that will be triggering the sensors by reflecting light from an LED. There are 2 sensors that will be at slightly different angles so that you get 2 outputs about 90 degrees apart.

If you supply the 12V to the signal PCB and turn the spindle, you should see the outputs change up and down. If you want more detail, you could look up the sensors if you can identify the part number.

And make sure the circular PCB is clean, as dust and dirt could cause rogue signals.