This appears to be the Clipper/Compressor circuit...
Special thanks to all whom are listed in this thread...
There are "two" circuits on the board, one for Variable - the other for the "compression" - which uses a "Zener" and swamped with a resistor to reduce the clipping distortion "clamp" effects. Looking at the board, the Zener band appears to a 5.1~5.6V type which when you place it in line with a Clipper Diode already present from the output of the Mod transformer - the diodes "clamping" effect is on all the time. - only stops or becomes evident once your Zener reverse biases then the Resistor takes over until the Zener breaks down again.
The Cap on the bottom is the "smoothing" filter.
So the "negative part" is what the Zener reverse biases and stops clamping - shutting off the Directional effect of the Zener and allows the Resistor to do the work.
What keeps the Zener from blowing up is the Resisters voltage drop across itself - these two work to clip the audio waveform. But drop wattage thru both - so they won't take too much power so this could be an Achilles heel for the design. Be very careful with this.
And so you know, that what is shown on the eBay listings is not how you
Should install the board, if you wanted compression - the photo shows you a working method - but uses the "JP36" variable holes used for the 2nd part of the board, which is the actual variable.
Caveat Emptor!