In 1936 the US Patent Office declared Bell first on its list of the country's greatest inventors, leading to the US Post Office issuing a commemorative stamp honoring Bell in 1940 as part of its 'Famous Americans Series'. The First Day of Issue ceremony was held on October 28 in Boston, Massachusetts, the city where Bell spent considerable time on research and working with the deaf. The Bell stamp became very popular and sold out in little time. The stamp became, and remains to this day, the most valuable one of the series.
Bell's stamp is part of which line? (If the question is unanswerable, say "unanswerable")
Famous Americans Series