have an extra M105 + M7821 unibus card?

They used to be very common. Original Unibus SPC cards were dual height and fitted into connectors C and D of the slot. You put an M105 address selector in connector E and an M782 (or M7820, M7821) interrupt logic board in connector F The M105 is the address selctor. It decodes the address from the Unibus, generates SSYN as appropriate and supplies 4 decoded regsiter select lines and read/write lines to the peripjheral controller. The M7821 is the interrupt logic, it handles the bua master part of the interrupt sequence, supplies the vector, etc. I haev no idea where you'd find one now. 'In DrARD's junk box' is not a useful answer, although true. I also have (I think) some 3rd party dual height boards that fit into connectors E and F and perform both functions. Try looking in the printsets for early SPC devices (KL11, PC11, DR11-A, etc). Some of those should include the M105 and M7821 schematics. -tony