Interesting - not sure if that is what my Orac looked like when it was new! I bought mine from the estate of someone who had started to rebuild the control electronics although the evidence suggests that someone had already had a go at doing that. At least I had a pretty clean sheet to start with, and no hang-ups about "authenticity". Very much about ending up with a relatively modern system, not the clunky electronics from a previous era. Anyway, as far as I remember, mine just had a disc with a single slot on the end of the spindle which presumably generated an index pulse plus vague idea of spindle speed and position for screwcutting. Maybe there had been something similar to that PC disc once upon a time but no trace remained. Given that my chosen motion controller could do a lot better than that, I fitted a 100ppr encoder behind the spindle, mounted on a 3D-printed platform that picked up some existing tapped holes. This is driven by a toothed belt and pulleys 1-1 from spindle. Seems to work well, and potentially more reliable and accurate than the device shown which might well have represented the art of the possible when Denford did the conversion.