i carefully opened the wires on the back and removed the horse.
the area between the horse & wheel was caked with old silver polish that i mostly brushed off. i found traces of a gilt finish on the wheel. originally the whole wheel would have had a gilt finish (clearly not a top quality one). i have other old costume jewelry that shows similar wear patterns.
the horse does not appear to have been plated with anything at any time; the back has a nearly black age patina on the copper, since that area never would have been handled or rubbed against clothing, etc. the front of the horse has something of a light patina, but through handling, polishing, or other contact never got as dark as the back. it seems that it was intended to have a bi-metal appearance, a copper or bronze horse against a gilt wheel.
it is also now evident that the wheel & arrow were stamped onto sheet (so the wheel is a manufactured piece, not one of a kind). the negative space between the spokes was sawn out by hand. as you point out, there are visible saw marks & unevenness along the insides of the spokes. the outer edge of the wheel shows file marks.
seeing the wheel without the horse makes me think it was intended to have something in the center, the design looks incomplete otherwise. i still think the horse may be original, but perhaps there were other designs available--i have seen this type of assembly on both badges & ordinary unaffiliated costume jewelry.
i feel certain that the pinstem & clasp are original.
whether it's a badge, costume jewelry, or homemade frankenjewelry, it interests me & i feel i've gotten my $12 worth.