Right
Finally been into the office and chatted with Graham. (Dactal Nightmare designer). The very oldest systems (Amiga) used POTs to read angles (3DOF not 6DOF). After that it was all magnetic tracking systems (Polyphemus with the occasional Ascension) and PCs (486 I think).
I found some old Virtuality hardware (Dev systems). They are going to the Leicester computer museum soon. I will try and get access again to them before then and try and image their hard drives