The 1802 Half Dime is one of the classic rarities in United States coins thanks to a small mintage and a low survival rate. Breen (1988) claims the date was first "recorded" in 1863, when three examples were known, but according to David Davis, the first auction appearance of an 1802 Half Dime was in Edward Cogan's December 1859 sale. In 1883, Harold P. Newlin enumerated 16 examples, but according to Logan & McCloskey (1998, p. 9), he overlooked at least a few auction appearances. In 1935, James MacAllister claimed to know of 35 examples (Breen, p. 279). The definitive listing of 1802 Half Dimes appears in Logan & McCloskey (pp. 9-38), where David J. Davis chronicled 167 sales and appearances over a 140 year time period. By combining duplicate appearances and matching against Newlin's and Breen's lists, Davis confirmed a population of 35 examples. The majority of the known examples are in low grade. The finest example known is the About Uncirculated Garrett example, though there are some close competitors. No true Uncirculated examples are known.
Sources and/or recommended reading:
Breen, W. (1988). Walter Breen's encyclopedia of U.S. and colonial coins. New York, NY:Doubleday.
Logan, R.J. & McCloskey, J. (1998). Federal Half Dimes 1792-1837. Manchester, MI:John Reich Collectors Society.
Newlin, H.P. (1883). A classification of the early half dimes of the United States. Philadelphia, PA.