Duhhhh. My brain finally turned on. It is of course perfectly possible to run UNIBUS _A_ (where the SPC slots are) in 18-bit mode too - although the _RH11_ can't use it that way. But you won't be using the RH11 anyway, so who cares? Also, I took another look at the KS10 tech manual, and they do in fact use use an M9200 'thin' jumper (although it's mis-labelled "M9300" in the diagram - that diagram has a number of errors, including the "M8014" in the UNIBUS 'A' In slot - they must mean an M9014 [UNIBUS to 3 flat cables] instead) to link the two UNIBI together. Which answers the question of how the KS10 CPU gained access to UNIBUS A (where the device registers, interrupts, etc are) when it also had to be connected to UNIBUS B (for 18-bit data transfers). So I think all our questions are answerered (except for the -AB/-C difference issue). Noel