This is a hand written message that is only visible because this box has been sitting in my closet for so long that the glue dried out. Hiding a message like that is a bit... mysterious.
I assume that this is actually just part of the manufacturing process. Google translate tells me the first part is something about river salt in Japanese. The second part looks like a product ID and the last part is a date in YY.M.DD format. But if it needs to be there why not just print it like a serial number?
Is this ironic?