DG Nova 4

Hello people, I'm currently seeking information on Data General's Nova line of minicomputers. I saved a Nova 4 yesterday, and since I'm unfamiliar with DG hardware I have a few questions. Is the the Nova 4 bus backwards compatible with the bus on the Nova 3 (i.e. can I pull boards from the 4 and put them in the 3) ? When I remove cards from the backplane, do I have to insert something like a "continuity card" ? The system was in a custom "Medtronic" cabinet, and together with the CPU I got a few I/O boxes which might be useless without the medical equipment. I pulled one of the boards from I/O box 1, and I hope somebody can identify it: DMA0 PC BD 207 005-00 C ASSEMBLY 877-121-00 MDA0048-87 (handwritten) CC board, sevice, sys zaak The boards in the I/O boxes look just like the ones used in the CPU, same handles, same bus connector, but they're shorter. Furthermore, I'm looking for a description or picture of the back of the CPU box, since I suspect something is missing on the right side of mine. Last but not least, a description or picture of the top of a Perkin Elmer ST-2222 diskdrive would also be greatly appreciated (is there supposed to be a lid ?). Sincerely, Erik.