Bonds or other obligations of Puerto Rico or any municipal government therein, payable solely from revenues derived from any public improvement or undertaking (which revenues may include transfers by agreement or otherwise from the regular funds of the issuer in respect of the use by it of the facilities afforded by such improvement or undertaking), and issued and sold to the United States of America or any agency or instrumentality thereof, shall not be considered public indebtedness of the issuer within the meaning of .section 745 of this title