Click on covers to see other examples:
By Flying Boat
27k
Zeppelin Mail
58k
Posted at Sea
43k
Paquebot
35k

 
Flying Boat
This registered Gibraltar postcard to Brazil was supposed to go by Zeppelin airmail ('pour le dirigeable Graf Zepplin via Friedrichshaffen') missed the Graf Zeppelin and was sent by Flying Boat instead (note the cancellations).

   Information and scan kindly provided by John Cooper