When Luger Pistols were zeroed at the factory, a small mark (a line) parallel to the bore was placed (stamped) across the top of the front sight base and across the top of the barrel thickened edge to indicate the zero locations of both parts when the gun was zeroed. See the attached picture and look at the top of the left side (as facing from the back of the gun looking toward the muzzle) and you will see a deep witness mark! Barrels and receivers also shared a similar mark underneath the reciever when they were matched up! The line up looked better on the original picture! ~Thor~
http://boards.rennlist.com/upload/SightWitness.jpg