A stamp in one color can be elegant, but the Post Office and the public soon looked for the visual interest of a second color of ink. The first U.S. “bicolor” postage stamps with two colored inks were issued in 1869. For reasons of cost, the technique was used only sparingly, usually on high-value stamps. One of the most famous is the 24¢ 1918 airmail stamp of the Curtiss “Jenny” airplane.
Two-color stamp engraving at that time required two separate engraved plates, one for each color of ink. The center image, or “vignette,” of the stamps was in one color and engraved on one plate; the outer “frame” of the stamps was in a different color and engraved on a different plate. The paper went through the press twice, once for each color.
In the case of the Jenny, the use of two plates and two printing passes opened the door to a classic error. Separate plates were engraved for the red frame and the blue plane inside it. Almost all the stamps were printed correctly. But then something went wrong; either a plate was put in backwards after cleaning, or the paper was turned around before it went through a second time. The airplane was briefly turned upside down, or “inverted,” relative to the frame, creating the famous “inverted Jenny.”
Several sheets with the error were caught and destroyed, but one made it through to a Washington, D.C., post office. There it was sold to William T. Robey, in perhaps the most famous discovery in U.S. stamp collecting. To avoid a repetition, the word TOP was later added to each plate. It appears in both colors at the top of the full pane.
Die Proof of 1918 “Jenny” Airmail Stamp
Signed Plate Proofs of 1918 “Jenny” Airmail Stamp