If the time within which the validity of the bonds may be contested has elapsed, to-wit: Sixty days from the date of promulgation of the result of the election, bonds issued by any district shall be registered by the Secretary of State without charge, and shall have endorsed thereon the words:
"This bond secured by tax. Registered on this ___ day of _________, 19__," and signed by the Secretary of State, or by an Assistant Secretary of State, with the Great Seal of Louisiana affixed without charge. All bonds issued under this Part are hereby declared to be negotiable instruments under the law merchants and incontestable in the hands of a bona fide holder for value.