What Are These Holes

Was working on my rear seat and noticed these holes in the back of the tub. The ones near the rear tailgate have plugs in most of them. The one about half way back on the bed are on the same rights left plane as the ones closer to the center that actually mount the body to the frame. But the outer ones go thru the hat channel underneath but there is nothing even close to the hole to attach. Are they just drain holes?

 

I wish I could help more but, I don't think they're drain holes ? They are most certainly ' Factory ' , though .
I'm sure we will both be educated on they're purpose.

 

The ones in the bed area by the tailgate are for the rear seat mounting brackets, I think. The plugged hole and the one outboard of it should have captured nuts where the bolts, that hold the seat mount bracket, thread in to. I could be wrong. My later tub had these. Not sure if it holds true to an earlier tub like your 58.

Not sure about the one next to the driver seat.

 

Based on my studying them from the top and underneath, they don't look like added by someone. Too precise and exactly same on each side. They look like stamped thru and not drilled. The single one is mid point and like I said goes thru the hat channel below, exactly centered. And exactly same on each side. Really look factory to me.

 

Hope this helps. Photo from 56 cj6

 

That expansion it. Thanks