When the Continental Bank Note Company won the U.S. government contract in 1873, they inherited the existing designs and printing plates for the 1870-1871 banknote series. To identify their own work and meet contract specifications, they added small, minor design changes—the "secret marks"—to the plates of the 1¢ through 15¢ denominations. For the 24¢, 30¢, and 90¢ denominations, Continental did not deem it necessary to change the designs because of their low usage, leading to those denominations being printed from the old National plates.
