Actually, the 300/350/400/450 take a different box than the SH/SM. The component parts to the SM box are the exact same as the parts of the SH box, the little "L" shaped rear brackets are attached (spot-welded from the factory) in the different location.
The number series box has the "IH" embossed on the back face of the box, the SH/SM box does not.
I bought two surplus SM boxes when FARMALL Plant closed down, $5/each... should have bought the whole pallet box... 20 or so. One's on my '54 SH, other's on the '51 M. The box on the SH was rusted out all along the bottom on both sides after 25 yrs. The new box is 35+yrs and no rust yet, but it hasn't seen the dust & dirt the first one did.
Not a big deal to attach the rear brackets, then the supplier can make a "generic" box for both SH & SM. I'd rather have the box made from thicker or better steel and attach my own rear brackets than have paper thin steel WITH the rear brackets.