It is component-video/S-Video connector.It can be found on old video equipment (TV for example)
Further to what red says, the card originally came with a dongle that had what was called a TV-OUT.
It had the s-video and/or component (RGB) connectors on one end.
Can you find one now? Doubtful, unless the person you got the vidcard hung onto it.
This is the one I got with my 3870x2.