Alright, I am making a parallel reply to my other reply, because I went down into the dungeon and busted out my Antiquated Electronics Adapters Box Of Doom to look for some breakout cables. I knew I have a dickton of this type of thing, and I do. We might be getting closer.

Check it out:

This is an old GeForce 7950 with an analog TV out, was was the style at the time, and what I believe to be its attendant breakout cable. This card IIRC only supports S-Video and composite out. The connector on that is very similar to yours, with only one pin relocated. And the difference is, this cable also supports stereo audio output, which is what I believe the unused pins in your connector were probably for, although your cable does not have an audio jack on it. The port it plugged into may have supported it, or a version of a similar gadget using the same port did.

For comparison, this is an A/V input cable for an ATI capture card:

Same connector, opposite purpose! Note that the ATI cable also uses the audio pins, but this time for input. Now you see why identifying these things is such a PITA. Manufacturers just did whatever the hell, either repurposing or reconfiguring outwardly similar or even identical plugs.

To clue in the user, these cables actually have direction-of-signal arrows molded into the top. Does yours? I can't tell from the picture.