Ron Guth:
A curious aspect of the Eagles produced at the San Francisco Mint is the placement of the mintmark. On the majority of Liberty Head Eagles, the mintmark was placed on the reverse just below and between the talon and the fletch of the arrow (for example, check out any O-Mint Eagle). However, San Francisco became an exception right from the start, when it placed the mintmark on the 1854-S Eagle further to the right, just below and between the fletch and stem end. This is true on virtually all of the No Motto $10 Liberties, though one 1856-S variety is an exception and shows the "normal" placement of the mintmark. Beginning in 1866, the "S" mintmark becomes very small and is placed in the "normal" position, as well.