Looking at the Seatguru diagram and the cabin photo, there appears to simply be a bulkhead separating the back of 6A-B from the door L2 behind it. Looking at this photo of the exterior, You can see a bit of a gap between the last window in WBC and the door.
http://www.airliners.net/open.file/1133439/L/
The NW Seat map for the 333 actually reveals the likely truth. Although it is a little small in resolution, it appears 6A is set back such that one window is missing and you would need to lean forward from the seat to see out the window.