Which bolts fit into these empty bed bolt slots? bed floor, in corners, near cab

I drive a 2010 Tacoma TRD Sport. the position of these empty bolt slots is perfect for D rings for a large crate I sometimes travel with. they are beneath the rear windshield, next to the bed-frame bolts. The underside of the bed shows a metal bracket beneath these empty slots which would be plenty of support for my crate. If anyone could tell me what size bolt will work, I'd really appreciate it. Looked everywhere on here and only found answers for the bed bolts that go into the frame. I do not won't to mess with those and don't have the tools for it anyway. Please look at this picture (not my truck) for reference. Thank you

 

Those are drain holes

 

Probably drain holes for water.
I have a tool box in my bed, or I’d go look.

 

I have them in my 2012 as well. Drain holes.

 

Drain holes 100%

 

As others mentioned, they’re drain holes for the bed so they’re not threaded. If you really feel like it, you could use a bolt with a nut underneath, but I’d strongly advise against this because that part of the bed isn’t structurally supported and you’ll most likely tear out the bolt and damage the bed if you put any reasonable strain on an attached d ring. Really though, why would you want a d ring next to another d ring? Seems excessive. You’re better off strapping to the corner loop or bedrail.

 

They are NOT threaded.