Boss CS-2 Compressor Sustainer pedal, made in Japan 1983. This is one of Boss' most revered pedals from the classic made-in-Japan era. They use a different circuit to both the earlier CS-1 and the later CS-3. The transparent tone, fast attack and lively response have made this a favourite on many boards for several decades. This one has a few marks but is in pretty good shape. 

Model: Boss CS-2  Compressor Sustainer
Made: Japan, April 1983 (seiral 299900)
Type: compressor
Power: regular 9v supply or battery
Mods: none
Accessories: none
Condition notes: few nicks and scratches, but good condition

Here's a good demo to give you the idea.