[saxman]

"I wonder why anyone would put 'witness marks' on the barrel and receiver if they were already installed and lined up. What purpose would they then serve?"
Simple, actually. As I explained, after the barrel was properly torqued and headspaced, the witness mark would have been made so the barrel could be removed and the sight and extractor cuts could be made in the correct positions. At reassembly, all the worker had to do was match the witness marks. I believe Jim's description of the various jigs and gauges is correct, but it seems to me that they would have been used at different points in the assembly. For example, the jig which holds the gun and makes the witness mark I think would have been used after initial assembly but before the sight and extractor cuts were made. There had to be such a jig to insure that the cuts would be made exactly 180 degrees from the mark. The weak point in Jim's argument is that the witness mark was put on the barrel before the barrel was threaded to the receiver. Even the use of the threaded gauge does not guarantee that the marks will line up when the proper torque is applied. I just can't see this being the correct sequence; it's imprecise and very 'un-German'!