Just because something is Numbers Matching...doesn't mean it's original...such as a carburetor that has the proper numbers but is a replacement to the car.
The 2013 Mopar Nationals form for OE Judged cars lists NOM as Numbers Original Matching.
Seems like NOM now is often seen as Not (Non) Original Motor.
So I would write out whatever you mean to avoid confusion.
I have seen NOM used to indicate all numbers matching. It is a numbers matching car, engine, transmission, rad support, fender tag, dash frame VIN tag...I'll change the post to avoid confusion....