 The zip code on Richard Cheslers (The Narrators boss) business  card is 198090 (shown briefly in the scene where The Narrator beats  himself up in his bosss office). In the shot where we see Tyler  Durdens business card (just before The Narrator calls him), The  Narrators thumb could possibly be hiding the 6th digit. All we are  able to see is 19808... then its all thumb.