Hm. So the DD11-B backplane does not appear to be SPC/MUD. Instead it
seems to handle 4 SPC devices along with a DF11 "communication line
adapter" in the middle AB slots.
That brings up an interesting question: How did this system ever work in
the first place? I know I ran it with two of these Plessy cards and a
RX01 controller but now that I look at it that would be impossible as
both were hex cards and both could never fit in a 4 slot backplane with
enough space for a quad spc
Maybe it did only run with 16kw of memory. Or maybe I had a 9 slot
backplane in there, but I don't remember getting rid of one.