Ron Guth: The U.S. Mint ended production of Twenty-Cent pieces for circulation in 1876, but continued to manufacture Proof coins for collectors through 1878, after which the denomination was discontinued forever. For some reason, the Proof mintage in 1878 jumped up slightly from the previous year, perhaps due to the introduction of the Morgan Dollar.
While the PCGS Condition Census for the 1878 Twenty-Cent piece includes PR66 and PR67 coins, the typical grade range for this date is PR63 to PR64. Cameo examples are scarce and Deep Cameo examples are rare. The finest examples certified by PCGS include a single PCGS PR67 and a single PR67 Cameo.