by the United States. For such purpose, such obligations may be acquired (1) on original issue at the issue price, or (2) by purchase of outstanding obligations at the market price. The purposes for which obligations of the United States may be issued under chapter 31 of title 31, are hereby extended to authorize the issuance at par of special obligations exclusively to the fund. Such special obligations shall bear interest at a rate equal to the average rate of interest, computed as to the end of the calendar month next preceding the date of such issue borne by all marketable interest-bearing obligations of the United States then forming a part of the public debt; except that where such average rate is not a multiple of one-eighth of 1 percent, the rate of interest of such special obligations shall be the multiple of one-eighth of 1 percent next lower than such average rate. Such special obligations shall be issued only if the Secretary determines that the purchase of other interest-bearing obligations of the United States, or of obligations guaranteed as to both principal and interest by the United States or original issue or at the market price, is not in the public interest.