Both of the 20 dollars were struck at the San Francisco mint, hence the S beneath the eagles tail. The first year for the 20 dollars was 1849 however this date is unique, the only example residing in the Smithsonian, from 1850-1907 the liberty head design (as yours) was struck during this period at 4 mints, Philadelphia (no mark below eagles tail), Carson City (cc beneath eagles tail), New Orleans (O beneath eagles tails) and San Francisco (as Mervyn's pieces)in 1907 the design was changed with standing Liberty and a left facing flying eagle, the last date struck was 1933 when the US went off the gold standard, only one of the 1933s is available to own and it sold some years ago at $6 million dollars. While there are several rarities in the series, in the main, unless they are in absolutely mint condition they tend to be worth little more than their gold value. All the best, Paul