The 6NZ (C-15 engines) were started in the year 2000. You just have to remember that OEMs (Kenworth, etc.) may have 100-300 engines at their factories of 3406E (2WS) at the start of the year 2000. They install those before they start on the newer C-15 engines, but they still call their KW truck a "year 2000".

Just look at the BIG "prebuy of engines" all the truck OEMs did, before the year 2007. Everyone was afraid of the more expensive, newer emissions engines(of any engine manufacturer). I heard as long as the OEMs didn't go over some "normal inventory rule" that the EPA allowed they to stock up on a certain amount/volume of engines before DEC 31, 2006. Then install those "prebuy" engines in 2007 built trucks/chassis.