This fascinating series of U.S. issues comprises 250 different stamps with just five Scott illustration numbers. During a period of extensive experimentation in printing and perforating technology this series developed using different papers, flat, off-set and rotary presses, plus the introduction of coils and booklet stamps. The result is a fantastic specialist area, or just an essential area for reasonable completion of a general collection. For further reading on the subject, The Washington-Franklin Heads: Simnplified! by: Larry S. Weiss is an excellent source. Published as a reprint by the Bureau Issues Association, it is highly recommended reading, even for the generalists.