The port is called DMS-59. It was used to make more compact video cards, etc.
From DMS on wikipedia,
"DMS" stands for Dual Monitor Solution. "59" refers to the 59 pins.
DMS-59 is generally used for computer video cards. It provides two DVI or VGA outputs in a single connector. An adapter cable is needed for conversion from DMS-59 (digital) to DVI (digital) or VGA (analog), and different types of adapter cables exist. The connector is four pins high and 15 pins wide, with a single pin missing from the bottom row, in a D-shaped shell, with thumbscrews.
You can view or buy a compatible adapter on amazon
With some electrical ability (wiring, soldering), you could use the pinout to make your own adapter. You would need to buy a DSUB-9 to wire it as a VGA