[lugerholsterrepair]

Mike, I believe the nub you speak of is the stop on the bottom of the barrel. The witness marks would be towards the muzzle end a quarter inch or so, This is the portion of your pistol that has been ground off.
There is actually no reason for witness marks on your barrel. These were ostensibly to align the sight to the top dead center. Yours has no sight to align.

Why would someone do all these things? The longer you look into Lugers you will find many bored machinests did this and much more. Why? Because in days past these pistols were worth much less than they are today and were a machinists playground. Just as the thousands of 98 K rifles that were converted into hunting rifles. Now worth a couple hundred bucks but if not monkeyed with are worth thousands.

As far as your serial # goes..It's difficult to argue one way or the other because they cancel each other out. Anyone capable of machining and installing this barrel certainly has a set of number dies laying around. Everything else is numbered 24..why not my new barrel? Who's to say?
Here's another thing to speculate on..The chamber got ground away because the bored machinest couldn't quite get a proper fit on the new barrel in the chamber area so the easiest place to take up the slack is to grind away the receiver untill it snugs up.
One can speculate untill the cows come home and it's mostly a guessing game of conjecture.

All I know is no competent gunsmith would willingly grind away that much of the receiver, part of the structural integrity of the original design. Not no how, no way. Bubba did it.

Jerry Burney