By 1877, the failure of the Twenty-Cent piece had become evident, and the Mint quit making the coins for circulation. However, collectors could still acquire Proofs from the Mint, at a small premium over their face value. Demand for Proofs was even lower than in the previous year (by more than half), and, in fact, the mintage of the Proof 1877 Twenty-Cent piece is the lowest of any date in the series with the exception of the rare, branch-mint 1875-S Proof.
Alone, PCGS has certified 60% of the original mintage, doubtless with some duplications, but the high percentage reveals some interesting information about the quality of the survivors. The most common grade is PR-63, followed by PR-64, indicating that collectors were not particularly careful with protecting their Proofs. Gems are very scarce, and none have been certified finer than PR66 by PCGS -- in any format. Non-Cameo Proofs make up the bulk of the surviving population. Approximately 20% of the survivors are Cameos, and only two examples have earned the Deep Cameo designation.
All in all, the Proof 1877 Twenty-Cent piece in Proof is a great coin for a several reasons: it's a Proof-only issue, the mintage is low, it's an unusual and popular denomination, and it's hard to find in top condition.