When I first dug this I thought it was a reins guide, but it doesn't have a threaded post so it can't be. There are no holes in it so if it was attached to anything I'm not sure how. It has an interesting chain design around the outer edge.
You might sift some of the soil around the find location. If it did have a lens that was resin mounted ground moisture might have decayed the resin over time.
The lens if any would be significantly rainbowed by now. Of course there's always the possibility that the lens was lost or damaged and the frame was discarded.
You might try copying the ornate pattern around the rim. Even if you have to draw it so you can interpret if flattened out has some significance. Pic three seems
to indicate that the "holding tab" is not quite perpendicular to the ring? That might indicate which eye it was designed to enhance.