b. The service in the police force of any member of such force, to be eligible for the additional pension credit under this section shall be terminated and he shall be retired on or before the first day of the calendar month next succeeding his attainment of age sixty-two provided, however, that in the case of the chief of such force who has attained the age of sixty-two on or before September first, nineteen hundred sixty-six, to be eligible for additional pension credit under this section, his service shall be terminated and he shall be retired on or before August thirty-first, nineteen hundred seventy. Notwithstanding the foregoing, during the period terminating August thirty-first, nineteen hundred seventy, such chief shall be retired on the first day of the calendar month next succeeding his attainment of age seventy; provided that service rendered by such chief after attaining age sixty-five shall not be used to provide additional benefits, except that the salary paid for such service may be used in the computation of benefits.