| I haven't been able to find a concrete hardware reference online for any card from that time. It was possible to run both a 2D only and a 3D only card. So, from what I recall it's either a header for an optional daughterboard/bay device for video capture, a pass through cable from another video card, a CCIR656 port, or perhaps for manufacturing.
I have a RIVA 128 with a similar header and the documentation makes no mention of it.
You shouldn't need any other cables to get standard VGA functionality from the card by plugging the analog monitor cable directly to the bracket port. | 
Linear Mode
