+3 on the not 18th Century...not sure how old but agree with posts above. That and here in Wisconsin (I can't be more than a county away) something that old would be exceptionally rare.
I went back to the same area of my yard today where I found the buckle and dug this up:
It is a piece from a 1940s hernia truss, like this one:
I'm starting to think the buckle came from the hernia truss and the leather straps have disintegrated. I can't seem to find an image of the exact truss though, so I can't compare the buckles.
EDIT, I've attached the pics since inserting them didn't seem to work right.