National Currency was changed around 1907 replacing the red seal and overprint with blue. Series 1902 Blue seals are the most common large sized national bank notes found today. While common from large banks and cities, blue seals from less populous states can be worth thousands of dollars. Western states like Nevada, Arizona, Alaska and Idaho are always in high demand. Blue seals come in two varieties as seen on the back. Plainback and Dateback. The $5 and $10 dollar banknotes below are plainbacks while the rest are datebacks. Datebacks are the older design and can occasionally carry a slight premium.