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 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, if such average rate is not a multiple of 1/8 of 1 percent, the rate of interest of such special obligations shall be the multiple of 1/8 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 obligations of the United States, or of obligations guaranteed as to both principal and interest by the United States or original issue at the market price, is not in the public interest.