The debentures issued under this section to any mortagee with respect to mortgages insured under section
1709 of this title shall be issued in the name of the Mutual Mortgage Insurance Fund as obligor and shall be negotiable, and, if in book entry form, transferable, in the manner described by the Secretary in regulations. All such debentures shall be dated as of the date foreclosure proceedings were instituted, or the property was otherwise acquired by the mortgagee after default: Provided, That debentures issued pursuant to claims for insurance filed on or after September 2, 1964 shall be dated as of the date of default or as of such later date as the Secretary, in his discretion, may establish by regulation. The debentures 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 as are issued in exchange for property covered by mortgages insured under section
1709 or section
1713 of this title prior to February 3, 1938 shall be subject only to such Federal, State, and local taxes as the mortgages in exchange for which they are issued would be subject to in the hands of the holder of the debentures and shall be a liability of the Mutual Mortgage Insurance Fund, but such debentures shall be fully and unconditionally guaranteed as to principal and interest by the United States; but any mortgagee entitled to receive any such debentures may elect to receive in lieu thereof a cash adjustment and debentures issued as hereinafter provided and bearing the current rate of interest. Such debentures as are issued in exchange for property covered by the 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; and such debentures shall be paid out of the Mutual Mortgage 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 that the Mutual Mortgage Insurance Fund fails to pay upon demand, when due, the principal of or interest on any debentures issued under this section, 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.