In every county in which a retirement system was established prior to January 1, 1952, the prior service pension is an additional pension for members purchased by the county or district, equal to one-sixtieth of the average annual compensation earnable by him during any three years of service elected by the member at or before the time he files an application for retirement, or, if he fails to elect, during the three years immediately preceding his retirement, multiplied by the number of years of prior service credited to him.