holes in 34 pickup cab.

I have some square holes in the back of my 34 ford pickup cab. I have noticed these hole in other cabs. What are they for? I plan to fill them in but wanted to make sure I did not need them.

 

I pretty sure they are for the carriage bolts that hold the wood inner structure in.

 

They are bolt holes for the wooden seat back.

 

They are to hold the wood inside cab I believe for reinforcement, I am also thinking of covering the holes up but I was thinking of welding a stud to hold wood,we made our own wood but I think macs antique parts carry them .hope this helps and good luck on your build Rudy