[jan-jaap]

It looks like there is a tact switch on the board (at N104)? That's pretty strange.

It's a reset switch. It's unusual indeed. There are some other things that don't make perfect sense:

• The board was designed to work with the Professional IRIS backplane. It has a WD SCSI controller that maps to the same user-defined pins on the VME P3 connector as the IP4 does. It interfaces correctly with the control panel on the drive bay tower, the 7 segment LED works. The reset switch there works too. I haven't tried the DIP switches, but expect those work as well.
  
• But it does not use the ribbon cable to connect to the keyboard / serial ports on the bulkhead, like IP4. You've got the same ports you'd find on the E-module of a 4D/25: keyboard, 2x serial, parallel and AUI (without retaining clip), but they're internal to the system
  

I don't really know how the graphics were supposed to work. There's an installation guide for the V30/V35 (a 6U VME set) on Bitsavers which indicates a ribbon cable was used to bus P2 from the CPU to the graphics, perhaps a similar mechanism was used here. Even if I had a TG-V20 graphics board I'd be reluctant to just hook that up. They probably made a 9U VME version of that too.