Deposits of securities provided for by the regulations in this part shall be made with any Federal Reserve bank or any branch of a Federal Reserve bank designated by the Office, or the Treasurer of the United States, and shall be held subject to the order of the Office with power in the Office, in its discretion in the event of default by the said self-insurer, to collect the interest as it may become due, to sell the securities or any of them as may be required to discharge the obligations of the self-insurer under the Act and to apply the proceeds to the payment of any benefits or medical expenses for which the self-insurer may be liable. The Office may, however, whenever it deems it unnecessary to resort to such securities for the payment of benefits, authorize the self-insurer to collect interest on the securities deposited by him.