Toronto "741" "821" and "821 A" Air Mail Handstamps

In the late 1930s, the Toronto post office applied a three-digit numeral handstamp on some air mail letters addressed to foreign  destinations. I have seen three different handstamps, "741" , "821", and "821 A" on covers mailed from 1937 to 1939 . Unfortunately I have been unable to find the reason why the numbers were used.

741 

The only 741 handstamp I have seen has serifed numerals.

 

 April 1, 1938 (Station "K")

Air mail to Curacao, Dutch West Indies

 

  May 26, 1939

Air mail to Tucson, Arizona

October 1, 1939

Air mail to Tacoma, Washington

821

The 821 handstamp exists serifed and sans-serif

Serif

August 11, 1937 (Toronto Station "A")

Air mail to Des Moines, Iowa

 November 8, 1937

Air mail to New York then South Africa

 

 December 15, 1937 (Toronto Station "A")

Air mail to Honolulu, Hawaii

Sans-serif

 

 

December 11, 1939

Air mail to Colorado Springs, Colorado

821 A

The only 821 A I have seen is a sans-serif handstamp.

October 31, 1939

 Air Mail from London, Ontario (Toronto transit) to London, England

September 8, 1939

Toronto to Bombay, India

821 A struck out with pencil