My opinion is that the side panels were more for style than any practical purpose. 

There were undoubtedly problems with overheating in some situations and that is why the panels were removed. If you ever had to repair the tractor or perform maintenance, the panels had no way of being held open, so they had to be removed. It would have been real easy to leave them off. Have you ever tried to put oil in a Fleetline and held the panel up with your head?

Maybe your father removed the panels during hot weather and put them back on in the winter.  Just my thoughts, WB.