All bonds shall be signed by the treasurer and countersigned by the chairman. The signatures of the treasurer and the chairman may be by facsimile, but said bonds shall be authenticated by such signatures as the bond resolution or indenture may require. All interest coupons shall bear the facsimile signature of the treasurer. In case any officer whose signature or countersignature appears on the bonds or coupons ceases to hold his office before the delivery of the bonds to the purchaser, his signature or countersignature is nevertheless valid and sufficient for all purposes the same as if he had remained in office until the delivery of the bonds.