Got to really looking at the frame of my F4 and it look like someone has beefed it up. Is this a common practice? A plate inside the frame itself and added a plate to the bottom of the frame.
Is this stock looking spring packs for an F4? Looks like a set of overloads have been added as well.

 

Looks like the frame is boxed. Possibly due to prior damage in the frame or for more rigidity. The springs are stock. The upper set of springs are overload helpers.

 

"Fish plates" was a common way to add strength and rigidity to frames. What you have may be damage repair or modification for strength. Can't tell from pictures.

 

My 48 F6 grain truck that came from MN has a similar type reinforcement - flat plate steel bolted to the outside of the frame running from about the axle to the cab.

Not oem, mine is obviously a homemade effort to support the crude addition of a hoist under the bed.