EAD DSP-7000 III- INV switch function?

I currently own an EAD DSP-7000 MkIII DAC. It has Series III faceplate and HDCD plaque over the de-emphasis lamp.
Does the INV (invert polarity) switch function on this model? I have tried this switch on about 30 CDs and can't hear any difference.
Thanks for any info.