The bonds shall be dated, numbered consecutively, and be signed by the president of the board and the treasurer of the district and countersigned by the secretary of the district and the official seal of the district affixed. The interest coupons of the bonds shall be signed by the treasurer of the district. All the signatures and countersignatures may be printed, lithographed or mechanically reproduced, except that one of said signatures or countersignatures to said bonds shall be manually affixed.
If any officer whose signature or countersignature appears on any bonds or coupons ceases to be an officer before the delivery of the bonds to the purchaser, the signature or countersignature either on the bonds or the coupons, or on both, is nevertheless valid and sufficient for all purposes the same as if the officer had remained in office until the delivery of the bonds, and the signature upon the coupons of the person who is treasurer at the date of the bonds, is valid although the bonds themselves may be attested by a different person who is treasurer at the time of delivery of the bonds.