There are nuts that just didn't get any threads cut in them!
I've seen a non-threaded nut four times in 58 years!!!
Two bolts in the same time.
So, take the t-nut out and the screw out and see if they will go together, just to remove this 1 in some number, from being the problem.
Check the threads on the screw also.
Once doing a diff rebuild I had a like problem until I removed one of the two springs LOL I had forgot that I didn't removed the spring and tried to put another one in the diff!!!