Canada: 1944 Air Letter with "Onwards Air Transmission" Instructional Handstamp.

Canadian Armed Forces air letter form used for civilian communication.

The air letter was originally posted unpaid on 26th December 1944. The postal authorities marked the air letter with "Returned for Postage" and returned it to the sender.  The air letter was posted again, with the correct 10 cents franking for India, on 28th December. This is a bit difficult to see, as the two machine cancels overlap.

Also, the air letter received a "O. A. T" (Onwards Air Transmission") handstamp.

"It is thought that an OAT cachet was applied to the top letter of a tied bundle of up to 60 letters. These bundles always travelled in open mail bags to a single intermediate or final destination, but where the quantity of mail was insufficient to warrant a closed bag. Closed mail bags did not attract OAT markings, as the whole bag was sealed and all letters went to the one destination as marked on the outside of the bag. Open mail bags were resorted en route as required and bundles replaced into other bags for the next section of their journey."

(Source: The Philatelic Database, http://www.philatelicdatabase.com/great-britain/onward-air-transmission/)