Sforno on Exodus 26:24/ספורנו על שמות כ״ו:כ״ד

1אל הטבעת האחת, to the ring at the thickest point of the board forming the corner post. [we referred to this in verse 23 where these two boards are described as serving with one cubit in a westerly direction, completing the length of the structure and with the half cubit constituting its thickness closing the gap left on either side of the six boards forming the western wall of the Tabernacle. (compare Rash’bam there) Ed.] The bolts (bars) running alongside the walls were also inserted both in the lower level and the higher level (above the center bar) through the rings attached to these corner boards at the south-west and south-east of the Tabernacle respectively, ensuring that the structure was solid.