An insurer may invest in bonds and other evidences of indebtedness which are obligations of any state, county, city, town, village, municipality, district, or other political subdivision of any state or of any instrumentality or board thereof or of the United States of America issued to provide funds for public projects, or for refunding of bonds issued for such purposes, which are revenue producing and self-supporting if such obligations are secured by a lien on such revenues to pay principal and interest and the issuing body is required to charge adequate rates for the services so provided to pay all charges against the project, including principal and interest on all indebtedness outstanding against the project.