Weatherproofing threaded holes under bed caps?

Im in the process of weatherproofing the bed and noticed these threaded holes under the bed caps.

What is there purpose? Does anyone know the screw size that could be used to plug these holes to prevent water/dust from entering? The threads already have some light rust and I would like to prevent further damage

 

Aluminized duct tape. If you just run bolts in them they will probably seize unless you antisieze the threads.

 

Just pipe some butyl sealant from hardware store into each hole. I wouldn’t bother adding hardware to them as it may not fit under the rail caps and might even add a galvanic corrosion element.

They are probably for the OEM 3rd gen tonneau cover.

 

If you put bolts in them you will see a bulge in your bedrail caps from the bolt heads. Just fill them with some silicone sealant.

 

Those are for the OEM tonneau bed cover, the latches bolt into them.