In the "Data and Maps" data that is included with ArcGIS Desktop, the zip code points file, 'zip_usa' (or for example, the layers referenced here: http://www.arcgis.com/home/item.html?id=1eeaf4bb41314febb990e2e96f7178df ) used to have a field called "ENC_ZIP".
For point zip codes (such as P.O. Boxes or single buildings that have their own zip code), this told you what zip code the point zip could be found inside.
When I look at current versions of this data, that field has been removed.
Does anyone know where this data went? Is there any other good source for this information?