I Assume the Case is Metal?
The Unused Terminal is Also Connected to the Shaft that goes to the knob and the Nut to hold it in place.
If that mounting Shaft is bolted to a Metal Case, that is the other wire.
As Long as there is Always Infinitive Resistance across the Capacitor, as you turn the knob, it is Probably OK. There is Not much that goes wrong with these caps. (Unless you Bend the Plates so it Shorts out.)
Without Knowing What this device is, and ALSO Seeing a Schematic of it, It is Impossible to really tell you much more.
Just because the Analogue Gauges don't change when you turn those caps, That could be any one of a Thousand Different problems. But Probably Not those Cap