Apparently there were two versions of the USA made Ross Compressor. One [perhaps] early version with the designation of “Sustain and Output” and the [perhaps] later version labeled with “Sustain and Level.” If you have more information on versions of the Ross Compressor (USA as well as Taiwan) please consider commenting in Disqus. I am pretty certain that the LED shown in the photo at left is a modification to the original design.
“It provides extremely smooth and creamy sustain and
accompanies well with overdrive pedals. Pedal is fairly simple to
use by adjusting the two knobs (sustain and level).”
More information here.