Sforno on Exodus 26:29/ספורנו על שמות כ״ו:כ״ט

1ואת טבעותיהם, it is customary for bolts to be inserted through rings unless the Torah would explain that it was not so, as in the case of the central bolt which was not fastened to the outside of the boards at all but inside of them as explained in verse 28.