@xssfox More fallacies:
- Each address will only have one postcode. Nope, The whole town of Büsingen am Hochrhein has German code 78266 and Swiss code 8238. Similar for some towns on the border between Germany and Austria.
- Each address will only have one postcode per country. Again no. In Germany it's common for places that receive a lot of mail (some companies and even some university dorms) to get a special postcode in addition to the one their neighbors have.