It's simple:
Under the PROGGSC pcb is the V4ROM. Eprom equivalent to the 27c160. The method is to mount one on top of the other, in this case it is one under the other sharing the same pins (1-1, 2-2,...41-41, 42-42) except pin 13. Pin 13 of V4 goes to pin 7 of IC 139.
The V4 window is touching...