The cancel date for this stamp is one day before the economic conference. I am assuming this is a philatelically contrived item marking the last day before the event, and the cancel being on the back because, well, it's a beautiful stamp. Unless, the stamp was printed on the glue, and the cancel on bare paper. The question remains, is the glue intact? And furthermore, if it is intact, on which side is it?
More, this could have been on the seaplane along the route they were using/experimenting with to fly airmail to Ottawa from the UK, in fig. 10 of this pdf. ( http://www.aerophilately.ca/stlaw.pdf
) you can see the seaplane and flying boat travel past the area that your cancel is probably from (Sackville N.B.).