Joesph Brady, a silver historian, kindly lent me his advice! Wow. Our local recommended antique appraiser couldn't even figure it out!! See his comments below:
Thank you for contacting us. There is no need for metallurgic testing of your tea & coffee service: It is definitely American coin silver, made by the Gorham Mfg. Co., Providence, Rhode Island, between 1865 and 1867. The 410 mark is Gorham's style number. It is lovely and rather valuable, and should be appraised. Prior to 1865 the lion would have faced left, and the G would have been in a simpler block style. Further, in 1868 Gorham, looking to market wares in England, adopted the British sterling standard -- 925 parts per thousand, as opposed to the US coin standard of 900 parts per thousand -- and introduced a date marking system.
The idea put forth that without the word "sterling" it is not valuable is quite ridiculous, since such marking is not seen on American solid silver wares prior to the 1860's. The earliest and most valuable pieces of American silver, such as the Paul Revere table spoon that brought $16,000 at Christie's in January or the New York-made punch bowl circa 1710 that fetched $ 5.9 million last year at Sotheby's, were marked only with the maker's own mark. The much-misunderstood phrase "American coin silver" actually describes all solid silver wares made in the American Colonies/United States prior to the commercial mining of silver in the U.S. in the late 1850's. It defines a period which began with the first Colonial silversmiths in the 1630's, when coins made up the majority of raw material available.