EmuDetector
Forum Supporter
No idea what it might be, but it appears you couldn't make it any worse. I might try scrubbing it with 0000 fine steel wool under running water. That might bring out a bit of the metallic shine, and possibly an outline of detail.
Try this too. Lay a piece of aluminum foil on top of the coin. Then, with a soft rubber pencil eraser, press down on the foil over the entire area of the coin. Any bit of detail will rise and show itself in the foil. Stay away from dried up older erasers, as they are too hard and don't conform to the latent design below your foil.
To get a better idea of the potential of this method, try it on a newer coin with lots of detail and an older one that is VERY worn, but still identifiable.
Whatever methods you end up trying, please report back with results.
Jim