The debentures issued pursuant to the exercise of an assignment option shall bear interest at the going Federal rate at date of issuance. The going Federal rate means the annual rate of interest specified by the Secretary of the Treasury as applicable to the 6-month period which includes the issuance date of the debentures. The Secretary of the Treasury shall determine this applicable rate by estimating the average yield to maturity, on the basis of daily closing market bid quotations or prices during the month of May or the month of November, as the case may be, next preceding such 6-month period, on all outstanding marketable obligations of the United States having a maturity date of 8 to 12 years from the first day of May or November, as the case may be. If there should be no outstanding marketable obligations of the United States having the 8 to 12 year maturity at the time the Secretary of the Treasury is required to determine the debenture rate involved, the obligation next shorter than 8 years and the obligation next longer than 12 years respectively shall be used.