You're in England, so I'd assume it's a local counterfeit, rather than an American one.
Counterfeiting was very common in the late 1700s. A majority of the halfpenny "coins" circulating in the 1780s were either counterfeits, or "evasions" - medals designed to resemble coins, but with some design or text changes to try to get around anti-counterfeiting laws.
Counterfeiting was of course illegal; numerous convicts despatched to Australia were convicted of forgery. Some of those forgers even helped make Australia's first coins after they got here, putting their coin-making skills to official use.
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