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if it turns out to be a Machin's Mills like I hope... tough to pinpoint whether it is a machin or a halfpenny
They're ALL halfpennies, in essence... whether British regal issues, British-made contemporary counterfeits or acknowledged American and Canadian-origin types (such as Machin's pieces).
That said, no, this is NOT the 1775-dated Machin's Mills (1775 is easy - there is only one die pairing). It is also not a "regal" halfpenny... but rather a garden variety contemporary counterfeit, presumably of British origin (there are more contemp ctfeits of this date than regal pieces). There are attempts to catalog and die/punch-link these... but long story short, this wouldn't be considered any kind of "interesting" example, I don't believe.
Hopium is a dangerous yet unfulfilling drug... If you're going to attempt cherrypicking Machin's in the wild (which is not impossible), you should know them cold or at least have a ready reference.

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as it was marked as
Seller was either "ignorant" (likely for the sake of plausible deniability) OR fully knew it was just a common $5-10 British CC but marked it Machin's thinking a bargain hunter would get all excited about it at that price - and that's exactly what happened.