when i lived in edinburgh in the 70s, we would just stick a card on the flat door with the surnames of the inhabitants on it - the flats themselves did not have numbers.
Norwegian apartments are all identified by a single letter and four numbers. For example, H0203 means 2nd floor, 3rd apartment from the left as seen when ascending the stairs. The H means "main floors" (as opposed to the loft or cellar). Very useful when visiting other apartments.