Looks more and more like a Data-East Laser Disc based system to me. The I/O card must be the DOS system with the IDC connector leading to JAMMA/I/O. The Capture card must go to the laser disc's output and the DE-9 port must connect to the Laserdisc's serial port for control. Could be a prototype or just a rare/unknown version.
On a quest for Digital 573 and Dancing Stage EuroMix 2