How to clamp a signal?
simulate this circuit &ndash; Schematic created using CircuitLab

It could very easily just be this. A shunt 8V zener diode. I don't know your frequency or voltage parameters. Is there a power/voltage rating on this little box? Is it custom or is there a mfg. part number?

Edit: There is going to be a limiting resistor in there somewhere. If you measure the resistance from your IN to your OUT port, you might actually measure the current limiting if it's in series. Or, you could apply a known voltage and measure the current draw to complete your schematic.