This discussion is courtesy of PCGS Coin Facts.
The following narrative is from "Silver Dollars & Trade Dollars of the United States: A Complete Encyclopedia" ( Wolfeboro, NH: Bowers and Merena Galleries, Inc. 1993):
Availability of Proofs: Today, Proofs are readily availabe (within the context of the trade dollar series). However, nice quality Proofs seem to trade less frequently than the population data suggest. In 1992 a leading dealer sought to put together a date set of 1873 to 1883 Proof trade dollars, and found that the 1876 was the last coin acquired, and that 1873 wwas also difficult to track down. The rarity of 1873 is to be expected, but the elusive nature of the 1876 came as a surprise.
Most Proof are of the 60 to 63 level. Many of the 1876 proof coins were handled carelessly. So this coin offers a fairly unique opportunity.
