Debentures issued under this subchapter shall be executed in the name of the General Insurance Fund as obligor, shall be signed by the Secretary, by either his written or engraved signature, and shall be negotiable. All such debentures shall be dated as of the date of default as determined in accordance with subsection (d) of this section, and shall bear interest from such date at a rate established by the Secretary pursuant to section
1715o of this title, payable semiannually on the 1st day of January and the 1st day of July of each year, and shall mature twenty years after the date thereof. Such debentures shall be exempt, both as to principal and interest, from all taxation (except surtaxes, estate, inheritance, and gift taxes) now or hereafter imposed by any Territory, dependency, or possession of the United States or by the District of Columbia, or by any State, county, municipality, or local taxing authority. They shall be paid out of the General Insurance Fund, which shall be primarily liable therefor, and they shall be fully and unconditionally guaranteed as to principal and interest by the United States, and such guaranty shall be expressed on the face of the debentures. In the event the General Insurance Fund fails to pay upon demand, when due, the principal of or interest on any debentures so guaranteed, the Secretary of the Treasury shall pay to the holders the amount thereof which is authorized to be appropriated, and thereupon to the extent of the amount so paid the Secretary of the Treasury shall succeed to all the rights of the holders of such debentures.