Is it possible that a EuroDIN was installed at that position, and the plastic housing around the pins was removed/broken off and the soldered pins remained? I think I see a keying hole on the circuit board in one photo where the bump from the EuroDIN housing would go.

Also, is that PLCC socket the right number of pins for an FPU? There were some FPGA chips which came in 84 pin PLCC packages. It's unlikely, but the upgrade could use an FPGA for its GLUE logic and somebody might have salvaged the FPGA out of the socket at some point.

I think this latter possibility is unlikely because there appear to be four PLDs on the board to serve the GLUE function.