I was thinking that Ford went to that body style in 1952 but the link below says '53.  They did keep that body style through '56 (my uncle had a '54 and then a '56) and made a change in '57 (I have one of those).  I believe '57 was the first design away from the step-side for Ford.  Chevy went there in '55 or '56.

The link says they went to the F100 - F360 series in '53.  I don't recall - need to check the model number on a '56 dually I have access to (edited to add that the '56 dually is an F350.)  You model appears to replicate a taller box such as a ton single wheel model had.

http://en.wikipedia.org/wiki/Ford_F-Series#First_generation_.281948.E2.80.931952.29