It's listed in the FAA registry as a manufacturer's temporary registration number held by Boeing. Some of the 737s are painted at the Renton factory. Some are flown a few miles away to a paint hangar at Boeing field. I suspect they use this number and others for the short flight to the paint hangar or any other flying before it is painted. Once it's painted it would have the customer's registration number on it, but still could be flown for acceptance or test flights with a different temporary N number issued to Boeing.