I strongly suspect that the gunk on the coin has ment it's gone undetected over all these years , even the dealer missed it .
It is in fact probably one of the great rarities of the bronze farthing series - its much rarer than the 1860 mule , and on a par with peck 530 - thats why it goes for silly money in any grade , I am sure only 50-100 exist and most are in well established collections .