My guess is the po cut a hole to access the captured nut for the k member bolt because it spun. It's happened to me before and had the weld the nut then weld the access back up.
I agree if you look closely at the oblong hole on the inside of the rail you can see where someone used a holesaw to start the hole and looks like they hit the inside brace and stopped. the round hole on the outside of the rail is factory the oblong hole has been cut out by some PO