Find a MoToR's or Chilton's manual that covers 1950's cars...that will have a section on OD trannies...the operation is pretty-much standard on all 1950 & later US OD set-ups, which is almost always a Borg-Warner R-10 unit.
The cable & knob you describe is the "Lock-out cable": this is used to manually disable the OD function. Usually, when the knob is pushed-in, the OD is "enabled"; when pulled-out, the OD is "locked-out".
There is also a governor on the OD section of the tranny, as well as a solenoid which engages/disengages the OD (causes up-shift/down-shift). There is a relay under the hood, which links the solenoid & governor to the kickdown control on the throttle linkage, and also the ignition.
If any parts and/or wiring is missing, the OD won't function.
If you get a reprint of the factory shop manual for your truck (Check American Classic or one of the other "specialists"), this should have service info on your OD.
Good luck !
