the spot welds you see are dimples to locate the center thingy, just put your trailing arms in to rotate them, they are tight, but they do move. sometimes i take a dead blow to them back and forth to loosen them up some more.
make sure that the threads are totally centered on the slot aswell
when i built those beams(i think those are the beams i built)
i checked to make sure i could thread in the set screws... but i might have missed one
i \"think\" they are m12