I found the resistor in the compressor pedal that set the current through the LED. It was a 4k7 so max current was about 1.9mA. Changing it to a 1k or 10mA effected nil change in performance, and annoyingly the most dramatic change in gain was still only when I fitted or removed the back cover. Actually depending on the gap or space between the cover and the housing I could vary the volume to various levels so the level control portion of the compressor works very well, just the level detection and LED driver section is useless.

After spending a few quality Sunday hours on this compressor thing and trying a number of alternate value components I've restored it to original and decided to set it aside. It works, but not as well as could be expected. Some mid winter R&D and subsequent modification is needed to turn it into something I think is useful...