Nice stamp, although in rough condition, 

regardless this rare stamp is super rare used 

showing an SON Ottawa AUG 31, 1918 cancel.

THE AERO CLUB OF CANADA'S 

FIRST AERIAL MAIL SERVICE - PER ROYAL AIR FORCE 

- AUGUST 1918 -

 

These stamps were required in addition to regular postage in order to be carried by air.

There were only 2800 of these issued on August 28, 1918, just one week 

before the last flight took place on September 4, 1918.