Pickup box ID and part needed?

[karasmer]

Say I am working on a 31 pickup and set the box on the frame and noticed how low it sits and from looking at some photos it looks like I am missing a Z type metal piece on each side that sits between the frame and box (continuous along each side) what is this part called. Next I searched box identification and there seems to be no agreement on what entails a 31 box, mine has a stake pocket approx. 7 inches in from the front end not in the corner? Thanks