The Pearl CO-04 compressor made in the 1980's by Pearl and is part of the Sound Spice series. Pearl pedals were built in Japan by a company called KLM (much like Ibanez pedals were built by Maxon) and were engineered to include some very innovative features for this era. 
The CO-04 was a much needed update on the compressor market in the 1980's. The CO-04 uses the classic CA3080 IC (famously used in the Ross and MXR compressors) and features a tone control similar to the circuit found in the Ibanez Tubescreamer. 
The Level knob controls the volume of the signal, the Attack knob increases the speed of the playing attack time, the Tone knob controls the frequencies of the signal and the Sustain knob controls the level of compression on the signal.