Longbed Stepside Cross Sills

[ChevyNewb]

Ok. so we got the truck with no wood in the bed. we are just putting on the bed now after paint. My question is the cross sills. the first 3 seem easy enough (counting the front stepside as one) we have the smaller/shorter cross sill that only mounts to the wood and then TWO more for the back end PLUS the rear sill. The issue is the smaller one. With it on the frame, there will be a hump, the gap is appropriate at all the other sills.
Just not sure if we have an extra sill or what? did the longbeds not use the small/thin sill in the middle?

[ChevyNewb]

Here are some before pics. with all the sills. the one in question is the smaller one in the middle. see the gapping would be off if we keep it in...