At the request of the Trust Fund, it shall be the duty of the Secretary of the Treasury to invest in full the amounts appropriated and contributed to the fund. Such investments may be made only in interest-bearing obligations of the United States issued directly to the fund.

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 directly 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 per centum, the rate of interest of such special obligations shall be the multiple of one-eighth of 1 per centum next lower than such average rate. All requests of the Trust Fund to the Secretary of the Treasury provided for in this section shall be binding upon the Secretary.