Overload springs
 

When i was in high school, my father purchases a new '66 Chevy c-10. It had "overload springs". The springs were essentially one half a set of leaf springs. they bolted to teh rear of the frame and curved down to the rear axle support. As the load increases, the leaf springs made contact with the rear axle support, thus assisting with the load.

this is a long drawn out story, but the question is, Does anyone have a pair of these for sale?