Here in The Michigan Boonies, most of the metal hits appear to be (I followed a couple of detectorists around the yard here one time, letting them know the location of the vanished farm buildings) broken rusty bolts off things, broken bits of flat metal pieces of things (sometimes with bolts through them), bits of barbed wire. In the west barbed wire was in use here and there, but where the grass was a little lean (and on the altiplano) herd grazing tended to be mobile and managed by riders and probably dogs, so, not much in the way of wire, and thanks to the distances covered, lost and dropped items would be spread out over perhaps hundreds of miles.
I often run across old barbed wire fences on my hikes in CA, ID and MT. I often wonder who put it there and when. The stuff is rusted, but seem indestructible. I saw a fence line with five strands of barbed wire last week — an unusually high number of strands.