No service shall be included under this part for which a member of the Defined Benefit Program is entitled to receive a retirement benefit in a lump sum or installment payments, for other than military service, from any public retirement system other than this system, or under the American Gratuity Act No. 4151 relating to service in the Philippine Islands under which 15 or more years of creditable service has accrued, or the San Francisco Employees’ Retirement System. If a retired member under this part becomes entitled to that retirement benefit, his or her retirement allowance shall be reduced thereafter to exclude the service upon which the retirement benefit is based, without other change in his or her retirement status. This section shall not apply to any retirement benefit received from a defined contribution plan that is qualified under Section 401(a), Section 403(b), or Section 457 of the Internal Revenue Code.