I am sure that all will be relieved to know that this will be my final comment on the "lines" issue. Those who dismiss anything but their own ideas as "nonsense" will not be interested and need read no further.
With that understanding, please permit me a summary of the theories, as I understand them, put forth so far to explain the barrel and receiver markings usually known as "witness marks" or "draw lines".
But let me dispose first of one secondary issue, the contention that barrel and receiver were, for whatever reason, finished, blued separately (rust blue or tank blue), and then (re)assembled. This idea is simply not supported by the guns themselves or by the practice of any other gun maker of the time or since. Luger barrel and receiver threads are "white" where the bluing did not reach; the parts could not have been blued separately, certainly not by the tank bluing method. Further, Mauser blued barrel and receiver as a unit in its rifle work, and there is absolutely no evidence that they or any other maker did differently with the Luger.
Now, let's look at the main barrel-receiver alignment theories.
1. The barrel and receiver were completed except for the barrel front sight and extractor cut. The incomplete barrel was installed on the receiver and the receiver flats used to align and machine the front sight. The assembly was then marked, with a single line and using a single tool, to ensure proper re-alignment on final assembly. The barrel was then removed, the extractor cut was milled, and the barrel re-installed, using the marking to re-align the barrel.
Comment: This is perfectly possible, and there is nothing in the guns to contradict it except that almost all of the alignment marks appear to the eye NOT to have been made with the same tool, and mircroscopic examination indicates that different tools were used even when the line appears continuous. Further, it is inefficient, requiring extra time for disassembly and reassembly. In addition, each assembly would have been unique, and the fitting of spare barrels would have been difficult. (In this discussion, it should be noted that spare barrels have the line, which would not be the case if it were applied only in provisional assembly of production pistols.)
2. The tooling used in Luger production was precise enough that barrel threads could be cut so that alignment and crush fit would be possible without markings. The front sight and extractor cut were machined into the barrel in the barrel making process and the barrel was installed in the receiver. Any minor alignment adjustments were made by aligning on the front sight base.
Comment: This is also perfectly feasible, and is used today, but would require precision machining not easily achieved in production equipment, especially in the early 1900's. But this technique would not have required any "witness lines"; there would have been no need for them and no logical explanation for their presence. Even if they were applied to completed pistols for some unknown reason, there would have been no reason to put them on spare barrels.
3. The Luger markings in question are "draw lines". One is placed on the receiver in its manufacture, and the other is placed separately on the barrel, using a precision gauge tightened down on the threads. The barrel line is then used as a reference point to machine the front sight base and the extractor cut. The completed barrel is installed, being "drawn up" so that the two lines meet, and the assembly is complete.
Comment: This technique is suited to the production environment, and is known to have been used in other arms factories of the general period. It achieves the purpose without precision machinery. Further, it appears to be confirmed by the gun themselves, which indicate that the barrel line and the receiver line were made by different tools and were made while the barrel and receiver were separate (there is no "crossover" in the marking, and microscopic examination shows distinctly different tool marks even when the lines appear to the eye to be one continuous line).
In summary, I believe the "mysterious" marks were made in the process described in paragraph 3 above. In the absence of any detailed description of the process by someone who was actually there, I will accept that explanation.
Jim
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