This odd piece of kit is a strip of weld mesh, suitably cut and rounded off with a grinder.
It's purpose is to lie across the top of a rack and extend its length to hold up the end of a sports bag. I made it up for the purposes of endurance rides of a day or two where I need to carry some kit, but don't want the drag of panniers.
In the photo below, which is not using this extender, you can see how the blue bag sags over the end of the carrier. This obstructs the tail lights, and also prevents using the horizontal bar at the back of the rack for attaching a light.
I've wrapped the weld-mesh in old inner tube to stop it rubbing through the bag I carry, and I've made this extra thick at one end. This allows me to attach tail lights behind the bag by making it almost as thick as a seat post.
I lie it on the top of the rack so that it will support the whole length of the bag I am using, and then use cable ties to keep it properly aligned. I use a couple of octopus straps which go under the body of the rack to hold the bag and extender on the rack.
The only issue with this set up is that softshell bags tend to lose their shape and slide around a bit. I've recent bought a Kathmandu Circuit bag which has a firmer base; it's actually the bit that's designed to go on your back. With a couple of strategic cable ties, this keeps the whole set up more stable. (2017)