Promises made on banknotes have changed over time. Interesting blog looking at these changes.
Most members of the public don’t know how the underlying monetary systems work, but they do see what is printed on its banknotes. To the public, the morphing set of promises on the face of a Federal Reserve note would have been one of the more visible manifestations of a shift from a mish mash of paper currencies issued by two different institutions and pegged to gold… to one single legal tender currency issued by the U.S.’s now-dominant monetary institution, the Fed—the Treasury receding into the background.
Needull in a haystack 