You're absolutely correct, and I've fallen foul of my own criticism of assuming too much. Looking at the board the axis drives are presented onto the IDC headers (the 16-pin black connectors, 5-of) on the board. Okay, that gives us a bit of an issue to understand how these are wired.

I'm guessing there are at least 3 ribbon cables normally plugged into these (I think I did see this on an earlier piccy).

I'm going to change tack back to the e-stop, but I'll post this for now knowing that you're stood in front of the machine scratching your head. Give me 20 minutes to write up another thought.