There was uncertainty at the time of the First Flights, as to how FFCs would be sent from the end of the flight back to the addressee.
It looks like Mr. Jones wanted to be sure that his cover travelled on the First Flight from Botwood to Montreal, and returned on the First Flight from Montreal to Botwood. He therefore paid 10 cents for air to Montreal, and an additional 10 cents for the air mail postage from Montreal back to Botwood.
If this is what he wanted, he was successful. - The Montreal backstamp confirms his cover was carried to Montreal. The return flight left Montreal on August 10th, and arrived in Botwood the same day. This cover then continued by rail to St. John's, where it arrived on August 12th, and the cover was backstamped.
However, it turns out that the extra postage wasn't necessary. - I have a cover from Botwood to Montreal on August 6th franked 10 cents, that was also returned to Newfoundland by the Imperial Airways flight, and backstamped in St. John's on August 12th.