Air Mail Form From 1938

Here's something else I came across and I'm not sure of it's purpose.
Seems like something that would be given to the customer but there's no number assigned to it for any type of tracing or record keeping.
Measures 5" X 31/4" and the back is plain.
Another thing I was puzzled with is the printing in the top right corner the 3 could mean this was the third version of this form.
The A. M. S. would mean Air Mail Service and the last line has the quantity 600,000 and date 14th of April 1938.
However if you look at the stamp of the dispatching office it's dated Mar. 2 1936 two years earlier then the form.
Any help explaining this is appreciated.

Cheers, Bill

Could it be perhaps Canadian Forces Mail Ephemera ?

At least it could be the place to seek assistance.

My thoughts. A facing slip put on top of covers which were to be air mail lifted to Bancroft for delivery. Just one of no doubt several numbered cancelling devices used to identify the mail handler of the time at Edmonton terminal.