Said bonds bearing the signatures 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 of the municipality issuing the same. The validity of said bonds shall not be dependent or not affected by the validity or regularity of any proceedings relating to the acquisition, purchase, construction, reconstruction, improvement, betterment, or extension of the undertaking for which said bonds are issued. The ordinance or resolution authorizing said bonds may provide that the bonds shall contain a recital that they are issued pursuant to sections 91.620 to 91.770, which recital shall be conclusive evidence of their validity and of the regularity of their issuance.