"Greenback captures the unembellished functionality of the serial numbers that appear on the U.S. dollar. While the engravings on American currency have evolved continuously since the foundation of the Treasury, the same basic typographic forms have been used for serial numbers since 1928. Because each digit is printed by a rotating numbering wheel, every number in the series must be designed to occupy the same amount of space, making these “monospaced” or “fixed width” numbers. For the Greenback font, we preserved this consistent spacing, as well as a number of eccentricities (like the distinctive “open top” 4) that are characteristic of these serial numbers. Hoefler&Co. has refrained from giving Greenback a companion alphabet, to avoid tempting would-be counterfeiters...

This is most probably not a font