If you search XC-UJM in the photo database, it gives the MSN #. That's how I backed into it. The Boeing orders page might also have some info. It's like how UA had 744s with -451 as the customer code. They took over planes initially ordered by NW.
The best compilation and updated source of all kinds of information related solely to the Boeing 757 is Marcel van Noordenne´s website at the following link. Here you can find all kinds of information in well organized database of the venerable B757.