Yes, the rear cab mounts changed sometime about 48-49.
The earlier ones have a single rear mount in the center back of The cab.
Late cabs have a mount on each side at the back of the cab.
I have both a 48 and a 52, and they are indeed different.
The cabs can still be switched, but the right mounts need to be swapped on the frame. Probably simpler to put an early cab on a late frame, may only involve drilling two 3/8" holes. The other way around will take 4 holes and some brackets.
Fuel tanks moved from on the frame to inside the cab at about the same time, 48-49.
Big bolt cabs follow all the same rules as the small truck cabs, and in fact they are identical and swappable.
That's how I understand it as of now, please correct me if I am wrong,
Grigg
