The steel work looks like its basically a mounting point for a 3 point seat belt.....When you do a full side conversion and fit a rock and roll/apex type bed and have a rear wardrobe the old seat belt obviously useless so hence the welded bar...The new seat belt(or use the original you took off) can be attached to this steel frame.....
Look at ava and wellhouse pics and you will see the belt in position directly inline with the steel work!!
Not a big job really if you know someone who can fabricate one!!!
You just need to know exactly where the seat belt needs to be...
Hope that helps...GAvin..