I'd mark it and keep it for now. You might be able to identify it later. The full set of images is not on this link. But it maybe some where else? As far as me buying any of these, they may determined normal coins. But I would save them for now. Proof dies are only used to strike 3K coins. So they are limited in number. On business struck coins there could be a million, per die pair. So they would be more common. But I'd save them until they are decided what the future for these might be.