The difference between a United States Note and a Federal Reserve Note is that a United States Note represented a "bill of credit" and was inserted by the Treasury directly into circulation free of interest. Federal Reserve Notes are backed by debt purchased by the Federal Reserve, and thus generate seigniorage, or interest, for the Federal Reserve System, which serves as a lending intermediary between the Treasury and the public.
Legal tender status guaranteed that creditors would have to accept the notes despite the fact that they were not backed by gold, bank deposits, or government reserves, and bore no interest. However, the First Legal Tender Act did not make the notes an unlimited legal tender as they could not be used by merchants to pay customs duties on imports and could not be used by the government to pay interest on its bonds. The Act did provide that the notes be receivable by the government for short term deposits at 5% interest, and for the purchase of 6% interest 20-year bonds at par. The rationale for these terms was that the Union government would preserve its credit-worthiness by supporting the value of its bonds by paying their interest in gold. Early in the war, customs duties were a large part of government tax revenue and by making these payable in gold, the government would generate the coin necessary to make the interest payments on the bonds. Lastly, by making the bonds available for purchase at par in United States Notes, the value of the latter would be supported as well. The limitations to the legal tender status were quite controversial. Thaddeus Stevens, the Chairman of the House of Representatives Committee of Ways and Means, which had authored an earlier version of the Legal Tender Act that would have made United States Notes a legal tender for all debts, denounced the exceptions, calling the new bill "mischievous" because it made United States Notes an intentionally depreciated currency for the masses, while the banks who loaned to the government got "sound money" in gold. This controversy would continue until the removal of the exceptions in 1933.
We need to bring back United States Notes and retire the Federal Reserve System 100%. Banksterism is THE problem. Thaddeus Stevens was 100% correct.