My first thought was it might be, but the sharp edges around it made me think differently. It wouldn't be anything on the bridle at all with those sharp edges and I can't think where else it would go on the harness. The two slide things on the back keep pointing to something on the bridle as decorative piece for the edge of the browband (I'm still waking up and trying to think of the right word) but I have never, ever seen one, antique or modern with sharp edges and I have one that is well over a 100-years-old and is brass with a glass center piece, but is round and they only have ONE of those metal bridges in the back. I've never seen one with two before for the bridle.
I'm thinking it had to be something that was somewhat decorative and it would have needed to sit on where a cross piece of leather came under it to stabilize it in place. So I don't know, Tom what it was for.