The ordinance authorizing such refunding revenue bonds shall prescribe all the details thereof and such bonds shall be in such form and denomination, payable at such places, bear such date and be executed by such officials as may be provided in such bond ordinance. Such refunding revenue bonds shall mature within not to exceed 40 years from their date, and may be made callable on any interest payment date at a price of par and accrued interest or at a premium, after notice shall be given by publication or otherwise at any time or times and in the manner as may be provided for in the bond ordinance.
     Such ordinance may contain such covenants and restrictions upon the issuance of additional refunding revenue bonds, or revenue bonds for improvements and extensions to such utility as may be deemed necessary or advisable for the assurance of the payment of the refunding revenue bonds thereby authorized; such bonds shall be payable solely from the revenues derived from such municipally owned utility and such bonds shall not, in any event, constitute an indebtedness of such city, village or incorporated town within the meaning of any constitutional provision or any constitutional or statutory limitation and it shall be plainly stated on the face of each bond that it does not constitute an indebtedness of such city, village or incorporated town within any constitutional or statutory provision or limitation.
     The validity of any refunding revenue bonds shall remain unimpaired, although one or more of the officials executing the same shall cease to be such officer or officers before delivery thereof, and such bonds shall have all the qualities of negotiable instruments under the Law Merchant and the Negotiable Instruments Law.