Bed slats that hold up the wood.

[8door]

One of the problems I have in waiting so long to rebuild my truck,
is forget where what goes where.
I did bag and mark all the bolts and screws, and put tape on the wiring looms otherwise my feeble brain would shut down. I know none of you have ever had this problem.............
Anyway to the bed question : It is a long Fleetside wood bed and there are several channel cross slats ( see don't even know what to call those ) that all look the same, but one.
It is about half as tall as the others.
Where dose it go ????????