OK, so the MkII octup uses a 4070 quad 2 input exclusive OR gate. I don't have any, but I do have some 4077 quad 2 input exclusive NOR gate, the truth table for a NOR gate is exactly opposite that of a OR gate. Am I right in assuming it should work in this application, the octave will just be inverted?

EDIT: I also wonder what difference it would make if the fundamental extractor is fed into the 4070, which is then fed into the 4024. So using the first three outputs of the 4024, you would (pesumably) get the original note but squared (which would sound quite synthy compared to the output of the fundamental extractor?), then 1 and two octaves down. Would this be the case? If so I think I might prefer a square wave of the original note to a "nasty comparator fuzz". Maybe? Or would it be essentially the same thing?