Question about a control in the cockpit

[4396T]

Hi, I've owned a 1971 Cherokee 140 for about a year now and I have a question about a mystery control in my cockpit. The control is a rotary knob just to the left of the heater/defroster controls. I had always assumed it to be part of the cockpit heater. Yesterday I was under the panel and was surprised to see a very large reostat attached to the back of this knob. Is this control for panel lights? There is no label of any kind. All help appreciated.
Mark

 

[Avidflyer2]

 

[4396T]

 

[Avidflyer2]

 

[GM.]

 

[Ed Dartford]

 

[kf6iat]

 

[Raptor05121]

 

[GM.]

 

[Avidflyer2]