This is a custom encoder made by jrok. website and info: http://www.jrok.com/hardware/RGB.html
It is used to convert standard resolution arcade boards for use on TV's and it can also be used to get S-video out of some older home systems.
I originally bought this as an alternative to an S-video mod for my Genesis but I have since found a different solution.
This is in excellent working condition with all the leads and diagrams needed. These are $71.50 shipped new. I'm asking $50.00 shipped. PM if interested, or to make an offer.