“Guaranteed taxable bonds” means bonds secured by a pledge of the full faith and credit of the United States for the payment of the principal and interest on bonds issued under the authority of this division and on which the interest is not exempted from federal income taxes. Where the federal guarantees are for less than 75 percent of the liability, municipal, state, or private guarantees, bond insurance, or mortgage insurance shall be obtained in order that the amount of all such guarantees aggregate not less than 75 percent of the principal and interest on such bonds.