Not entirely sure which knob you're talking about so I'll give you a few and hopefully catch the one you mean.
A - Circular knob which rotates left/right and sprung loaded to center is the pilot control for 'hoist in/out', that's the old winch/hoist which comes out of the right side heater closet and can be rigged to winch through the center hatch, not the newer external hoist you see mounted outside the right crew station.
B - 5 position knob to select which cargo hooks (fwd, center, aft, tandem, all) are opened when the cargo release is activated.
C - 3 position switch (off, on, emerg) to enable hydraulic power to the rear cargo ramp or enable the emergency pilot ramp controls on the same panel.
D - The actual FADEC control panel forward of the ECLs.
If your ride was in the 80s I'm guessing the aircraft was fitted with the older 712 engines which didn't have FADEC, Can't remember what switches were in front of the ECLs where the FADEC panel is now, it's been a while.