Chen Hsiang notes that besides using watermarked paper, there are secret symbols on both front and back of every stamp. Even post office employees do not know what these markings are -- only the printer knows.
陳祥指出，除採用有浮水印的水紋紙外，每一枚郵票的正反面都有暗記，這些暗記連郵政人員都不知道，只有原廠的監印人員曉得。