In Oz, interestingly, instead of a number we have an image code.
in the selvedge one will see images of Koala's, the number of which
refers to the print run.

In a similar vein, I have seen "traffic lights" in the form of images that
match the subject matter of the stamp. The Canadian $2 Polar bear issue, for
example, has traffic lights in the form of four little polar bears, each
corresponding to the colours used in the printing of the stamp.

By the way, I consider this sort of thing as part of "selvedge art" and
have published an article on it at
http://www.rpsc.org/Library/selvedge/selvedge.html.