So I just discovered that there are three wildly different variants of the M7821 Interrupt Control card. More here, with images:
http://gunkies.org/wiki/M782_Interrupt_Control I'll have to dredge around and see if I can find circuit diagrams for them all; they are wildly different, the -C has many fewer components than the -B and -D, and the latter has a couple of delay lines. Noel PS: Sorry about that dis-directed message to Cindy; not quite sure what I did wrong there.