In any case or any class of cases arising under these regulations the Secretary of the Treasury may require such additional evidence and a bond of indemnity, with or without surety, as may in his judgment be necessary for the protection of the interests of the United States.

The Secretary of the Treasury reserves the right, in his discretion, to waive or modify any provision or provisions of these regulations in any particular case or class of cases for the convenience of the United States or in order to relieve any person or persons of unnecessary hardship, if such action is not inconsistent with law, does not impair any existing rights, and he is satisfied that such action would not subject the United States to any substantial expense or liability.

Nothing contained in these regulations shall limit or restrict existing rights which holders of securities heretofore issued may have acquired under the circulars offering such securities for sale or under the regulations in force at the time of acquisition.

The Secretary of the Treasury may at any time, or from time to time, prescribe additional supplemental, amendatory or revised regulations with respect to U.S. securities. The Secretary also may lower the minimum and multiple requirements for stripping marketable Treasury notes and bonds issued prior to March 1, 1993, through an announcement as provided in § 356.31 of this title.