The Continental Bank Note Company reused all the National Bank Note Company’s 1870-1871 Issue printing plates. To distinguish its printings from these plates, CBNCo engraved a small mark (termed a ‘secret mark’ in philately) on the plates for most denominations.
The 1-cent Franklin has its secret mark at the bottom of the stamp. It is the small horizontal dash in the largest of the ‘pearls’ immediately to the left of the numeral ‘1’. CBNCo most likely engraved this mark with a small amount of acid applied to the plate on each stamp.
Beginning on May 1, 1873, the Post Office Department set a new one-cent rate for postcards, which the 1-cent Franklin filled. The stamp could also have been used for some third-class mail, drop letters, or in combination with other denominations, to fulfill higher rates. CBNCo printed approximately 780,000,000 stamps for the 1-cent Franklin issue over a period of six years. National Bank Note Company had only printed it for three years but produced double the number of 1-cent stamps.