(a) No bonds shall be issued by the district until they shall have been approved by the attorney general.
(b) After the bonds have been approved by the attorney general and registered by the comptroller of public accounts they shall be negotiable and incontestable.

(c) When the bonds of an issue are thus approved and registered, the bonds later delivered by the district in lieu of these bonds, pursuant to § 60.337 of this code in connection with the exchange of registered for unregistered bonds, or unregistered bonds for registered bonds, or in lieu of lost or mutilated bonds, need not be reapproved by the attorney general or reregistered by the comptroller of public accounts. Nevertheless, the bonds shall likewise be incontestable, and except for the limitations resulting from registration shall be negotiable.