Mr. Rubin: You asserted that, "The lede correctly quotes 31 USC 5112, but that defines coinage and the requirements for minting coins, not for bills or notes, which are also US$." The statute that authorizes "United States currency notes", Section 5115 of Title 31 of the United States Code, is referenced in the section on "Banknotes".  And the statute that authorizes "Federal Reserve Notes", Section 411 of Title 12 of the United States Code, is also referenced in that section. Those references are correct and they are where they should be. Please do not confuse bills of credit (e.g., United States currency notes) with money (United States dollars). Please do not confuse bank-issued promissory notes (e.g., Federal Reserve Notes) with money (United States dollars).

The U.S. Constitution authorizes Congress "To coin money" and Congress has authorized the coining (minting) of United States dollars. The Constitution also authorized Congress to borrow money, and Congress done that by authorizing the issuance of bills of credit known as United States currency notes. Those notes represent debts payable in United States dollars. Similarly, Federal Reserve Notes are issued by the twelve Federal Reserve Banks and those notes are, by statute, redeemable in United States dollars. 

Each Federal Reserve Note identifies the issuing bank by the first character in the note's serial number. For example, the letter "A" represents the Federal Reserve Bank of Boston. A complete list (A through L) can be found in Federal Reserve Bank. The Organization Cetificate of each Federal Reserve Bank is on file in the office of the U.S. Comptroller of the Currency. Each certificate lists the names of the private banks that own and control the Federal Reserve Bank named in the certificate.

United States Notes and Federal Reserve Notes are not United States dollars.  Those notes are evidences of indebtedness, not "money" as the word money is used in the U.S. Constitution. Please understand the distinction between money and notes payable in money.