Cal. Government Code § 31664.3

This section shall apply in any county having a population in excess of 2,000,000. A safety member shall receive no credit for prior service subsequent to June 30, 1921, and prior to January 1, 1938, in calculating a retirement allowance pursuant to this article unless prior to January 1, 1952, he files with the board his written election to pay into the retirement fund six dollars ($6) for each month of prior service for which he claims credit. If a safety member files an election pursuant to this section and has accumulated contributions to his credit as of December 31, 1937, which with interest to December 31, 1951, are less than payments due for prior service credit subsequent to June 30, 1921, the amount due or balance thereof shall be paid by additional salary deductions in the amounts specified by the member but in no case less than six dollars ($6) per month. The total amount due shall be paid prior to the effective date of his retirement. If the accumulated contributions with interest exceed the amount due for prior service credit the balance shall be refunded.