That is the part number for that barrel.
To be a little more specific, it's a Federal Stock #.
And, BTW, to me, a "Frankenstein 1911" is one that has such features as beavertail grip safeties, added sights (Bomar, Micro, Millett, etc., etc.), beveled magazine wells (particularly the add-ons), and other "useful" features. It's right up there with putting fender skirts on your pickup truck.