During the period in which the Continental Bank Note Company printed U.S. stamps, the first-class domestic rate for a half-ounce letter was three cents. Because of that rate, Continental’s 3-cent stamp was used more than all of its other stamp issues. The 3-cent Washington comprised over two-thirds (2,610,000,000) of Continental Bank Note Company’s approximate total output of 3,909,662,000 stamps (including all issues and denominations).
The secret mark on this stamp is the heavy shading along the lower white borderline of the larger ribbon that appears below the ‘R’ in the word "THREE." On the earlier National Bank Note 3-cent stamp, that ribbon has a simple, crisp white outline.