Used from 1979 until 1981, this mint mark type causes probably more confusion than any other. Known as the 'Clear-S' for 1979 used in contrast from the block-S that had been used since 1974, then only two years later it was known as the 'filled-S' or 'type-I' for 1981. Ass to that the fact that many people have a lot of trouble discerning the difference in 1981 mint marks on proof coins, this mint mark resides in a fog of confusion for many.
Let's clear the air - this mint mark has a nearly perfectly circular top and bottom which are both the same size. That is the distinction that this mint mark type is unique for in San Francisco mint marks.
