Notwithstanding any other provisions of this chapter, debentures issued under any section of this chapter with respect to a loan or mortgage accepted for insurance on or after thirty days following August 2, 1954 (except debentures issued pursuant to paragraph (4) of section 1715l(g) of this title) shall bear interest at the rate in effect on the date the commitment to insure the loan or mortgage was issued, or the date the loan or mortgage was endorsed for insurance, or (when there are two or more insurance endorsements) the date the loan or mortgage was initially endorsed for insurance, whichever rate is the highest, except that debentures issued pursuant to section 1715k(f), 1715k(h)(7), 1715l(g), 1715x, or 1715z–3 of this title may, at the discretion of the Secretary, bear interest at the rate in effect on the date they are issued. The Secretary shall from time to time, with the approval of the Secretary of the Treasury, establish such interest rate in an amount not in excess of the annual rate of interest determined by the Secretary of the Treasury, at the request of the Secretary, by estimating the average yield to maturity, on the basis of daily closing market bid quotations or prices during the calendar month next preceding the establishment of such rate of interest, on all outstanding marketable obligations of the United States having a maturity date of fifteen years or more from the first day of such next preceding month, and by adjusting such estimated average annual yield to the nearest one-eighth of 1 per centum. Notwithstanding the preceding sentence and the following paragraph,1