Two available in fully working order. Currently they are in a custom-made metal enclosure and work fine. They are set to measure sum and difference but can easily be swapped to show Left and Right. The optional Alice PPM704 drive card is a dual-balanced input to BS spec. There is no illumination available (as far as we know) for this particular PPM meter. Zero adjustment is done via the front panel adjuster screw and full calibration details are available in PDF.