It may be that there was an intervening process between the TD and the '03 which called for a proof cartridge AFTER assembly - just don't know.

Distinction needs to be made between "proofing" and "guaging". While they needed to weed out any bad apples, they HAD to have pretty well satisified themselves that the barrel was sound before expending a lot of labor on it. No one expected a rifle to fail structurally after assembly.

As to timed threads, how else would you do it? The extractor cut has to line up - it isn't made later after they thought the barrel was 'tight enough'. They were clearly able to index the threading and machining of the parts such as that when the witness marks aligned, all was good.