Thats the plate holder. All it is is a vertical pipe bolted to the subframe using existing bolt holes. That was welded to a horizontal pipe and the steel strip (plate mount). I have a small 2 foot aluminum bar that goes through the horizontal tube and is held in place with a pin and a cotterpin holding the pin in. I designed it so that all i have to do is pull the cotter pin out, th eretaining pin pulls out and the aluminum bar pulls out.
It's for this bag, there are 2 straps in the back that really don't have anywhere to strap on to, so the bag has a habit of inching forward.
Eventually I want to build a set of hard bags and a quick release rack to drop them off.