it was implemented in 1995. i still have my complimentary postal
code directory 1995.
the 6-digit postal code, the first 2 digits is the last 2 digits of
the old postal code, e.g. 769114, the old postal code is
XX76.
the first 2 digits is based on a list of wat we call a postal
district. tat's how the terms District 9, 10 and 11 came
about.
it increases slowly together with the last 2 digits. for e.g.
yishun 76XXXX is 2776. ang mo kio 56XXXX is 2056.