Speedometer or cable problems?

Sorry to say it sounds as if the cable applied too much force on a already worn instrument. There aren't any snap rings, bearings or much of anything really. A steel shaft is held in place in the aluminum bore with only a brass collar. There is a steel thrust washer between brass collar and aluminum bore but you can bet there is wear / more movement there. The square drive port gets a good load of contaminated grease packed into it too. Pull the cable out and shove it back in again without cleaning either cable end or port might have been fine for the service tech when all this was 5 even 10 years old. 30 + years old is different.
In the end its kind of like old plumbing, we'll never know how much longer it might have worked had it not been disturbed. Consider that you also disturbed the left/right/up/down directional force applied by that cable attach also. How straight the cable is routed to the back of this instrument directly effects how long it lives. Like I said, It wasn't built like a machine.
And the odometer is direct gear driven. The speedometer is not. This is 2 instruments in one housing sharing only the cable that drives them.