It seves the purpose of aligning an FM Stereo Modulator used in low power FM stereo transmitters. Two tone outputs are available, the low tone has a secondary output that is 180 degrees out of phase with the primary output.
http://www.solorb.com/elect/stereoosc/index.html
This simple tone control can be used in may audio applications. It can be added to amplifers, used as a stand alone control module, or even built into new and exciting instruments.
http://www.aaroncake.net/circuits/tone.htm
Simple add-on module Switchable "Control-flat" option.
http://www.redcircuits.com/Page67.htm