Said bonds bearing the signature of officers in office on the date of the signing thereof shall be valid and binding obligations notwithstanding that before the delivery thereof and payment therefor any or all the persons whose signatures appear thereon shall have ceased to be officers. The validity of said bonds shall not be dependent on nor be affected by the validity or regularity of any proceedings relating to the acquisition or improvement of the system or systems for which said bonds are issued. The resolution or resolutions authorizing said bonds may provide that the bonds shall contain a recital that they are issued pursuant to this article, which recital shall be conclusive evidence of their validity and of the regularity of their issuance.