We can't use the data from Postnummerservice, see https://wiki.openstreetmap.org/wiki/FAQ#Why_don.27t_you_just_use_Google_Maps.2Fwhoever_for_your_data.3F and https://postnummeruppror.nu/ . Neither the street itself nor the addresses on it have any postal code in the OSM database though, so the app/site that shows you 421 43 must get that info from somewhere else.