VERY FINE. AN EXCEEDINGLY RARE PLATE NUMBER BLOCK OF THE 24-CENT 1918 AIR POST ISSUE WITH BLUE "TOP" ONLY. ONE OF THE GREAT RARITIES OF UNITED STATES AIR POST PHILATELY.
The first printing of the 24c 1918 Air Post issue was trimmed so that the plate numbers and arrow were removed, resulting in a straight edge at top and right. After the invert error was discovered on one of the first printed sheets, the Bureau of Engraving and Printing decided to retain the top selvage. It added the word “TOP” to the blue “Jenny” vignette plate, so that when the new sheets were printed the word “TOP” would appear in the newly-retained upper selvage. This would show the pressmen the correct direction of the blue “Jenny” vignette plate, to insure that the invert error would not reoccur. Shortly after the blue "TOP" was added, sheets were printed with the additional red "TOP".
According to Don Price's exhibit of this issue and research by Joe Kirker, only 22 blue “TOP” only blocks are recorded.