Liberty Bell cachet handstamp
A 'cachet' is a marking on an envelope that commemorates a special event. Cachets may be printed onto envelopes before or after the event. They may also be applied to an envelope by a rubber stamp or hand-written or hand-drawn. They enhance a cover's appearance and desirability.
Cachets are ordinarily privately designed, created, and applied. Since they are not markings that are applied by a postal administration, they are not an auxiliary marking and are also not to be used as a cancellation or postmark. Those uses are exclusively within a postal administration's domain.
The Philadelphia Sesquicentennial was celebrated during 1926, marking the 150th anniversary of the signing of the Declaration of Independence. The year was also the first year of a Philadelphia, Pennsylvania-Washington, D.C., contract airmail (CAM) route. This cachet was applied to airmail tendered for that route.