I agree with Ian on the round roof hatches; I have seen several period liberation pictures of F60L GS trucks with round hatch....and also Ford 3 Ton LAAT.
Like Ian and Grant I believe square vs round hatch is an early vs late thing. I am not sure if it has anything to do with air superiority, though. I believe it was more an ease of production thing.....the round hatch is largely made from wood with a canvas cover....much simpler than the square steel hatch with hinges etc.
The roofs without hatch.....you see them a lot when you start checking pictures closely. First I thought the closed roofs might have been fitted to non frontline-vehicles, like the C60X Machinery trucks, Wireless vehicles and medical vehicles........but, they also appear on 3 Ton GS trucks in 1944/45 liberation pictures.....both on Chevs and Fords.
Ian could be right here.....contracts were leading (?)
Alex
Here is a picture taken in Utrecht 6 June 1945, memorial D-day parade. To the left a C60L without hatch....to the right a Ford F60L with round hatch. source:
http://www.hetutrechtsarchief.nl