#2 - Proof mark (barrel):

#4 - Why front sight blade is removed when front barrel band is removed or installed:

My first theory about the process would be something like this:
1.  Barrel blank is tightened to spec in the receiver.
2.  Index marks added to barrel and receiver.
3.  Barrel removed and fixtured using its index mark for alignment.
4.  Extractor notch cut, rear sight holes drilled and tapped, front sight installed.
5.  Barrel re-installed in receiver, torqueing until index marks align.

While touring the NH Ruger plant a few years ago I saw a similar procedure being used to assemble rifles, which model I can't recall.  But I do recall the barrels being tightened in first using a torque wrench, index marked, removed for machining, then re-installed.  That procedure avoided hand-fitting of the barrels to get proper torque and alignment.

Now, the question is, has anyone seen original Krag barrels that were never in a rifle but have the extractor notch, sights, etc.?  If they exist, that blows my first theory.

Second theory:  You can buy 1903A3, M1 Garand, and even modern-made Krag barrels that are cut for sights, etc. and have the threads "timed" so that only some hand fitting is required to get torque and alignment to come out right.  So apparently the Springfield Armory already had thread timing down to a science in the 1890s.  So they could've done all the machining on Krag barrels prior to first installation and just left them a few thousandths long for minimal hand fitting at assembly.  That would be more suited to a military product where parts interchangability was important rather than a commercial product.

NPS detail on machining of Krag magazine door.

Here is the complete Part 2 of the article:

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It's interesting that the barrel is proofed before installing in the receiver and before having the chamber final cut for the cartridge, page 268, left column, near top.

The finish chambering is done before the barrel is installed in the receiver, page 268, middle column.

gunboat57:  Thanks for posting the 1899 Scientific American pages.  It certainly gives a lot of insight into the manufacture of the Krag at Springfield.  The barrel and receiver were certainly labor intensive!  I wonder if the barrels were proofed on a special arsenal action or if they were tested with their final action and disassembled for sight mounting and browning?  (I think this is unlikely without both the receiver and barrel being serial numbered to aid re-assembly).  It also bring into question when the receiver was proofed?