2nd Gen Non-Engine/Transmission frame bolts

[criggs]

I'm putting a gooseneck hitch in a 2001 2wd. There are 4 bolts 1/2 diameter in the frame above the axle on each side. Two are 8 inches in front of the axle center line and two are 8 inches behind the center line, mas o manos. They are in the depressed part of the frame. The bolts have serated shoulders and drive into the hole. They go to nothing. Why are they there? I can't find any evidence of a prior hitch in the pickup.