In 1968, inspired by Great Britain's own post codes, the Japanese postal service decided that they should do something like that too.
Japanese postal codes used either 3 or 5 digitals. Number-kun has a 3 digit postcode on his body.
To remind everyone to use, at the post office, signs with Number-kun smiling and pointing to the numbers on his own chest were hung up.
They had one simple request written around them:
Number-kun wants you to make sure your postal code is correct and clear.
Thirty years later, in 1998, postcodes were changed to be 7 digits. Number-kun had to retire then...