AO-3921-4, 2005, American Advances in Aviation, P80 Shooting Star, BW Pictorial, Add On Cachet, SC 3921, FDC

Add-on cachet is:

A cachet design added to a cover which was originally uncacheted. An add-on cachet should be identified

by maker and date (mine are stamped on the back with the date) so that it is clear that it is not contemporary with the cover.