Debentures issued under this section shall be executed in the name of the General Insurance Fund as obligor, shall be negotiable, and, if in book entry form, transferable, in the manner described by the Secretary in regulations, and shall be dated as of the date of default as determined in subsection (g) of this section, except that debentures issued pursuant to the provisions of section
1715k
(f), section
1715l(g), and section
1715x of this title may be dated as of the date the mortgage is assigned (or the property is conveyed) to the Secretary and shall bear interest from such date. They shall bear interest 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 as are issued in exchange for mortgages insured after February 3, 1938, shall be exempt, both as to principal and interest, from all taxation (except surtaxes, estate, inheritance, and gift taxes) now or hereafter imposed by the United States, by any Territory, dependency, or possession thereof, 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, in the case of debentures issued in certificated registered form, 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 out of any money in the Treasury not otherwise 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.