Such agent's control of the said Bridge shall be subject to the rights of the holders of the outstanding Delaware Memorial Bridge Revenue Bonds of the State issued (or in the process of issue prior to the effective date of this chapter) under the authority of § 318 of this title, and the state covenants that the said Bridge shall remain in the said agent's control so long as any of the said bonds shall remain outstanding or unpaid or until all said revenue bonds issued or in the process of issue under the authority of § 318 of this title have been paid in full or provision shall have been made for the payment thereof in the manner provided in the indenture between the State Highway Department of the State and Equitable Trust Company, dated June 1, 1948, and the supplemental indentures thereto, dated June 1, 1951, January 2, 1952, June 1, 1956 and June 1, 1960.