British, George III, Bank of England, dollar, countermark on 1795 Mexico City 8 reales

In the absence of any regular issue of silver coinage since the reign of George II, the quality of coin circulating the late 18th century was very poor, being largely made up of counterfeit halfpennies and unofficial copper tokens. The silver coins which remained in circulation were very old and typically worn flat. The Bank of England tried to remedy this situation by issuing Spanish silver coins with a countermark of the King's head applied, in 1797. The countermark was easily forged and the issue was therefore a failure. The coinage is best known today for its contemporary sobriquet: the head of a fool on the neck of an ass. 

Host coin VF+, countermark about EF, light toning. 

Spink 3765A