Now I have never seen anything like this, but it has to be some kind of stabilizer arm, it is way to flimsy to be a shock absorber, and it looks like an hydraulic line going to the unit the arm hooks to.
This is almost the same way it would look for an airride system, but we know it hasn't got that. The way they show it hooked up if the spring comes up it will raise the lever which will turn the pin it is mounted to, and with that line going to it, somehow they must of had some kind of hydraulic assist load leveler.
There used to be a lot of things they put on trucks, that never done the job they intended them to do, it usually took many deaths before they realized it, and they never would listen to the people that drove them.
My bet is stabbilizer arm, what do you think?