Identification of the 6¢ large banknote
Step 3 - Look at the ribbon to the left of the figure 6
About the addition of the shading (called the secret mark)
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.