Bonds or notes of the United States, or other obligations which are unconditionally guaranteed as to both interest and principal by the United States, may be pledged and deposited by the export warehouse proprietor as security in connection with bond to cover his operations, in lieu of the corporate surety, in accordance with the provisions of Treasury Department Circular No. 154, revised (31 CFR part 225). Such bonds or notes which are nontransferable, or the pledging of which will not be recognized by the Treasury Department, are not acceptable as security in lieu of corporate surety.