Well I finally got the gauge finished and built a "package" to protect it during shipping.
Since Butlersrangers originated this thread with the idea that the markings on the bolt could possibly indicate a plus or minus dimension from a "mean" that would help with final assembly, I will send him the gauge first. With the checking of many bolts we might be able to discern which if any of these marks may indicate plus or minus dimensions. BR has indicated the marks might indicate lots of steel, other may indicate the shift or which line the bolt came from.
With the .009 inch difference I have seen measuring 2 bolts with an "s12" stamp on the back I would not recommend to anyone that bolt swapping is OK. These 2 bolts may have measured the same when they left the armory but the one with the more extreme wear was shorter.
So I will pack it tomorrow for sending to BR. I will also make a printable chart for recording the info. BR's chart on the OP looks good to me, right now I would add a line for shift while closing the bolt, I saw .004 to .005 inches on mine. This shift may be universal on all of them, it will be good to know this. I would like a copy back from each member that uses this gauge to check his bolts. I recommend you do one a time to prevent mixing them up.
If you have a Krag that closes on the field gauge and is loose on that gauge, the info we compile here may make it easier to get the right bolt to get the headspace back to a better range.
Two videos on using the gauge. The first is longer and shows more details on packing, the second is from a better angle and shows more details on actual measuring.

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Any suggestions on details for the chart or the videos will be taken into consideration.
If you want to get in on this exercise send me a PM, with your email address and how many Krags you have to check, especially if you have any new old stock on hand.