The bonds shall be signed by the president of the board or by such officer of the district as the board shall by resolution authorize and designate for that purpose. They shall also be signed by the treasurer, and be countersigned by the secretary. The coupons of the bonds shall be numbered consecutively and be signed by the treasurer. All signatures and countersignatures, except that of the treasurer on the bonds, may be printed, lithographed, or engraved. If any officer whose signature or countersignature appears on the bonds or coupons ceases to be such officer before the delivery of the bonds to the purchaser, the signature or countersignature is nevertheless valid and sufficient for all purposes as if he had remained in office until the delivery of the bonds.