Many Trade Dollars made their way to China, and they where used as trading currency in this China. A few began making copies of this coin to sell (it's considered a form of art work in their country), and unfortunately these counterfeits made it back to the US. Some of these counterfeit Trade Dollars fooled the experts, and many used to be sold on ebay until collectors became informed.
Mint mark: None (for Philadelphia, Pennsylvania) above the D of DOLLAR on the reverse.
A different die was used for some Trade Dollars, namely 1875 and 1876, and is referred to as Type I Reverse and Obverse. This Type II Reverse is identified by a berry located under the right eagle's claws. The Type II Obverse is identified by the ribbon ends pointing downward.