The special purpose revenue bonds bearing the signature or facsimile signature of officers in office on the date of the signing thereof shall be valid and sufficient for all purposes, notwithstanding that before the delivery thereof and payment therefor any or all the persons whose signatures appear thereon shall have ceased to be officers of the department. The special purpose revenue bonds shall contain a recital that they are issued pursuant to this part, which recital shall be conclusive evidence of their validity and of the regularity of their issuance.