valdiman Very nice coin.
However, I believe it is not Real. I think you have a Contemporary Circulating Counterfeit made in a very early version of Sheffield Plate. This process involved creating a metal sandwich consisting of two thin layers of silver with a core layer of off metal. Over time the silver layers got thinner. Here I see a very thick (which means early) Sheffield Plate. Here is why:

There are two features - first the corrosion at the upper edge which clearly shows the outer layers. Second are the bubbles in the outer layers caused by subsurface corrosion.
The rarity of this coin is GREAT. I have never seen one, but the number of collectors interested is small. I think it would bring about $50 - at least from a specialist like myself who is only interested in it as a TYPE coin.