my AP ND is 5/7/03
My receipt and online message also said that it was 400+ days, and I believe most people from NSC get the same numbers in their recepits from NSC.
My AP was approved on Aug 05.
When I had called the service ceter (phones worked back then), they said that since I-131 is also used for other sorts of travel document (and some travel document does take that many days to process), all I-131 receipts have the same processing time frame.