Don,
I believe this has been written up before but...
Many parts and peices of the 39 - 46 cab are the same, but the 41 through 46 are not interchangeable with 39 and 40. The 41 - 46 cabs were longer. I believe this length is all in the foot boards and firewall because the doors fit my 1940 cab.
I also believe the difference in the back glass is how they are installed on the truck, not the size or shape. My 1940 cab has a pot metal frame and matching rubber gasket for the rear window. I think that is different for 42 - 46.
Bill Schickling
