Early Canadian printings of Avon books look identical to the American books, but feature a white circle instead of the Avon book number.
In the early 1950's, however, there was a series of Avon books published for the Canadian market which adopted a unique numbering sequence. The letter C was placed in front of a three-digit number. The earliest number I have seen is C752, and the highest number I have seen is C813.
There is no relationship between these numbers and their American counterparts. For example, Robert Bloch's The Scarf is Avon 494, but the Canadian release is numbered C800.