Not one of my favorite designs, but a pretty low serial number.
Legal Tender Notes were first issued in 1862, in order to help the Union finance the Civil War. Gold and silver coins were disappearing from circulation, and a new and different monetary medium was needed. They were not exchangeable for silver or gold coin, so some merchants were reluctant to accept them at face value. Even the Philadelphia mint would not accept legal tender notes in payment at par for the proof coins it struck for collectors. To buy these coins, collectors had to submit other silver and gold coins in payment.