[math-fun] Postmarks

You have probably received mail that has postmarks in the form of a square consisting of pixels, some of which are filled in. Nearest I can can count, the square is about 42 by 42 = 1764 pixels. If each pixel could be on or off, a lot of data could be encoded there! Does anybody know what sort of data is encoded there, and how it's stored? Bob Baillie