Bill has it right. The loop is to pull it out when you unload the wagon. The rod holds the sides together. The rod usually goes on the outside of the tailgate boards and also holds the top of usually 2 boards from pushing out when wagon is loaded. The loop makes it easier tp pull out and also acts as a stop for the end When you tighten the nut up you can hold the rod from turning by holding the loop.
Have used them many times on horse drawn wagons.
Do you guys know the correct terminology for them, so I can Google them? Thanks.