Well this is just a guess ! I would thing it was some kind of Cast Iron Harness buckle or Fastener . I will look more later when i get back from Metal detecting today good luck !!!
OK Here is My Guess I think it is some kind of Cast Iron Harness buckle that was bolted to a yolk or collar.The small opening would have had a thin piece of metal through it and screwed or bolted to the collar or yo;lk. Then the big opening would have a leather strap through it with the ability to lengthen or shorten the strap to fit.
Thanks Gare! I will follow up on that thought. So far, I have not been able to find one like it in any of my books, but I could very well have overlooked it along the line. What gets me about this one is how massive it is yet how delicate it is.