One stamp on each of the first 8 of 10 panes of the U.S. Celebrate the Century series is engraved. However the last two panes in the series do not have an engraved stamp. Whaaat???

The first government-issued stamp to depict an airplane was US Scott #Q8 in 1912. However, that is not the first stamp to depict an airplane. That distinction belongs to US Scott #CL2, an airpost semi-official stamp privately issued in 1991. It is known as Rodgers Aerial Post.