Its a leaf spring. I dunno if wagons used them but my boat trailer had springs just like it. A bolt held one end and the other end moved inside a bracket as it compressed.
i think what you got there is a handforged strap hinge from a barn or gate... sweet find!!! go back and see if you can find the other one... i found a pair about a month ago.. couple feet from each other.. did electralisis on them ,and there one of my favorite finds!!!