In 1963, the rising price of silver finally led to the abandonment of silver-backed currency. Although silver was not removed from U.S. coinage until 1965, the decision to discontinue the SCs was made earlier, and the last notes of the type were printed in late 1963. At the time, the $1 denomination existed only as an SC, so it was necessary to introduce a new $1 FRN to fill the void created by its elimination.
The first $1 FRN was dated Series 1963, and its design and wording conformed to those of the other denominations in that series. Unlike the $1 SC it replaced, the $1 FRN had the denomination spelled out behind the Treasury seal. But the engraved border of the face design was also completely redone, making for the most radical change to any denomination since the $1 back had been replaced for Series 1935.
With the passing of the SCs, the $2 and $5 USNs were the only notes now in production besides the FRNs, and they were printed in relatively tiny quantities. The BEP considered their ongoing production to be more trouble than it was worth; but the law required that a fixed dollar amount of USNs remain in circulation. In 1965, to address this problem, the Treasury adopted a new policy regarding the USNs. The $2 USN was discontinued the following year, and the $5 in 1968; but a new $100 USN was introduced, dated Series 1966. A small number of these $100's were sufficient to make up the required circulation of USNs; and since $100 notes had a much longer lifespan than $2's and $5's, the need for the BEP to print additional $100 USNs after the initial print run would be almost nonexistent. To further ensure that the $100 USNs would wear out only slowly if at all, they were in practice kept "in circulation" only in a technical sense; they were "issued" mostly to a Federal Reserve vault, and relatively few ever reached the hands of the general public. Thus, from 1968 on, the FRNs were effectively the only note type still current--though it was not until 1994 that the USNs were actually abolished by act of Congress (at which time the Fed's vaultful of $100 USNs, representing the great majority of all red-seal $100's ever printed, were unceremoniously shredded).
The elimination of the $2 USN had, in fact, removed the $2 denomination from production entirely, since FRNs of this denomination did not exist. But the supply of $2 notes had been so small for years that their loss was barely noticed by the public, and so it was not considered necessary to introduce a $2 FRN as a replacement.
Next: A last few changes