Correct me if I'm wrong but...

Maybe container twist locks?

 

That box looks like it would be overheight on a standard flatbed. If that's all they haul, then it would make sense.

Then on the other hand he may not have that thing tied down at all!

 

Could be bolted on with those fancy hooks. I forget what they are called

 

I have a feeling that step deck is crying out in pain from all that weight concentrated in the middle. Gots to be a tiny bit of angle towards the trailer wheels where there should not be any.

 

I would agree with you, we called them corners. Lock your corners now ye heah? Half the container drivers would know we are referring to that corner lock on containers.

 

Looks to me there are some extra brackets welded on to the trailer frame under the corners.
Can be seen by zooming in the third picture.

So I'll go with through the deck container type locks.

 

Shoooot!! A good driver don't need no stinkin' securement!! He prolly rides around with a full coffee on the dash with no lid too!!