Piper PA-28 question

I recently purchased a 1973 Cherokee Charger. So far, I am really enjoying
the aircraft, but I have a question that has gone unanswered. There is no
mention of it in the POH or the information manual that I have.

Beneath the throttle quadrant but above the fuel selector (on the right
side), there is a knob that I am unsure about. This knob rotates and turns a
marker that indicates "L" or "R". I believe that this may be a trim for the
vertical stabilizer but I am unsure of how and when to use it.

I could be totally wrong, so any advice on this would be greatly
appreciated.