Four years before Number-kun was even a thought, the United States of America were introducing their own postal codes, which they call ZIP codes. ZIP is short for Zone Improvement Plan.
Inexperienced postal workers would have a much more efficient time sorting what letters go where now. But the public needed to know about the change, and they needed to remember the change, and worse of all they needed to adapt to the change. So, what happened? They recruited a mascot!
I think that Mr. ZIP must be related to Number-kun. Do you see the resemblance?
The two of them have big eyes, round ears, wide smiles and only want you to write your postcode down.