This is a underbody spring assembly for a dump truck tarp system. This spring mounts to the arms that roll the tarp up and down. The springs pressure is what holds the tarp down over the load. Some tarp systems mount to the bed and some underneath. This one mounts underneath the bed and that's why its called an undermount or underbody. The link below is the exact brand that you have, although this one is missing the mounting plate. The second link is just for reference to show how these work when mounted on the truck.

This is a underbody spring assembly for a dump truck tarp system. This spring mounts to the arms that roll the tarp up and down. The springs pressure is what holds the tarp down over the load. Some tarp systems mount to the bed and some underneath. This one mounts underneath the bed and that's why its called an undermount or underbody. The link below is the exact brand that you have, although this one is missing the mounting plate. The second link is just for reference to show how these work when mounted on the truck.

References:

https://www.tarpstop.com/ez-over-underbody-spring-assembly

https://shop.mentordynamics.com/product/undermount-spring-set/