The entire period of service between June 30, 1969 and December 15, 1984 during which an employee was erroneously classified as eligible for federal Social Security coverage and for which the employee paid FICA contributions that were not refundable at the time the error was discovered shall be deemed to be service as a noncovered employee, notwithstanding that the employee has paid FICA contributions and retains federal Social Security coverage for that period, if the employee applies to the Board within 30 days after the effective date of this amendatory Act of 1993.