Just made a minor breakthrough; a random usenet post
suggested that the
MDP-1000 was just a rebadged General Automation SPC-12, and so it is. I
suspect the internals of the unit I have (which is badged as an "MDP-6650"
on the rear) are a bit different than either the MDP-1000 or the SPC-12
(and I'm no closer to finding answers to my IC identification questions),
but it at least gives me another avenue to explore...
(Bitsavers has a few items:
http://bitsavers.org/pdf/generalAutomation/spc12/)