Dan is spot on with the selector circuit.
I would also check/replace relays in the stepping circuit such as; STU, STD, STO, STE and DZ. If your driving 500 miles to this job, I'd certainly take new relays to cover both circuits. R10 48VDC 4 pole latching and 4 and 6 pole standard relays are for U.S. equipment of the vintage you describe.