[tubman]

It's a "Helper" spring. It was a coil spring that hooked around the ends of the existing transverse spring. The far end of the eye bolts shown would go through the hooks on the end of the device and the nutts holding them could be tightened to add more or less tension to increase the load-carrying capacity of the rear suspension.

IT's probably part of the "Trailer Towing Package" installed by the owner. We used to come across them from time to time when parting out old Fords.