A new denomination, the silver trade dollar, was authorized by the Coinage Act of 1873. This provided that a coin weighing 420 grains, 90% silver, be struck for use in the export trade. By comparison, contemporary Liberty Seated silver dollars weighed 412.5 grains. Produced in quantity from 1873 through 1878, the trade dollars were a great success, particularly in China, where merchants preferred silver to gold and would not accept paper money of any kind. Coinage would have continued except for the Bland-Allison Act of February 28, 1878, which authorized the government to buy millions of ounces of silver each year and resume the production of standard silver dollars (which had not been minted since 1873). The trade dollar was discontinued forthwith; however, Proof impressions were made for numismatists through 1883, plus a small quantity of Proofs distributed privately in 1884 and 1885, these last two being great rarities today.
Choosing a trade dollar for a type set is easy enough to do, the choices being a circulation strike, which requires some connoisseurship, or a Proof, most of which are sharply struck and attractive. Enough exist in both formats that collectors will easily find a nice example, except that MS-65 and better pieces are elusive.