If it is a boot plate, how do you propose that they affixed it to the bottom of the boot? Civil war-era super glue? I see no screw holes in that piece.
Unfortunately this is just one of the many examples of "who knows" type finds that show up here. It could be everything from a brake pad from a 1993 Special Edition Barbie Car to a broken piece from the English crown. Personally I think it is some sort of broken piece off a random farming implement. I doubt even the farmer who had to replace it could identify it at this point from that picture alone. Do all the standard tests, e.g., magnet, weight, etc. If it isn't iron and it might be something worth scrapping, throw it in a bin. If it is iron, walk to the yard of a neighbor you really hate and toss it on the lawn. Then wait around for the next time they mow and sit around and relax.
-mcl