canteen 160 tonomat

thanks to all help so far! i have spent most of the weekend working through the schematic in the manual, re- drawing a loom wiring diagram and identifying which wire goes to each molex plug connector...a lot of the wiring had been butchered by someone, with wiring missing, or incorrectly connected.
and tracing the PCB wiring against the schematic.
i have now ordered a full set of thyratrons, and the components to re-make the power supply which is missing.....I do have one question the selector buttons has a blank button, the bottom of the left hand row.....this is wired, and on the schematic it shows it as TH i have no ideal as to its function,and cant find where this connects to..
if anyone knows, perhaps they can inform me. thanks, bob harper