The key date for 20 cent pieces is of course the 1876-CC... the same way the Flying Eagle cent key date is the 1856, or the Barber dime key date is the 1894-S. Pretty much no chance of ever getting one.
If that's indeed beyond your price range, take an 1877 or 1878
And yes, I agree this has to be due to the CC mintmark's desirability (though possibly more were melted as well).
If that's indeed beyond your price range, take an 1877 or 1878











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