I found this in Gettysburg on private property with permission. First Day battlefield. Have found other shell fragments and some sabot pieces there from Hotchkiss shells. Think this is a sabot piece?
Sorry to have to answer, Waldo, this time I don't have happy news for you about your find. It is indeed a modern automotive wheel-balancing weight. I believe if you chisel the encrustation out from under the iron you'll see it is the flap/flange which holds the weight onto the wheelrim.