Looked at Butlers rangers photo again and I think I see fine grain at the "corner" at 3 o-clock and a thin line on the bolt face and maybe a little thicker along the edge from 5 to 6 o-clock. If this I am really seeing that then the original heat treating is not suspect. Really hard to tell with the photo quality.
If there was old bore solvent, grease and oil on a portion of the fractured surface you could think that it let go gradually over the years. Makes me want to think it went all at once due to an over load.
Was there also a photo of the guide rib?

I saw the red also but thought the bolt may have been lying around for years since it came out of a "junk" box. If it was from an outside part that was exposed to the elements rust would be a real indicator. On a gun not sure. Stored in a box at the back of a bench after it broke, the softer areas further away from the lubricant will rust first. A lot of questions would need to be answered first and better photos would be needed to say what happened.
I was asked for an opinion on that tractor axle, the machine had 60 hours on it, but not being there when it broke left enough questions as to whether it should have been a warranty issue. If the operator side stepped the clutch when the engine was wide open would be one thing or did it break while pulling a steady load? The machine was stuck up to the axle in wet sand. The owner was not the one operating the tractor so no one questioned really knew what happened. All I could go by was the appearance of the grain structure of the steel, it looked right.
For all the comments about Krags in the reloading books there are very few cracked bolts to be seen.

psteinmayer wrote on Dec 22^nd, 2017 at 12:47pm:

Not many examples out there, it looks like the one on top has a crack through the bright spot from the flash/light. Not bad considering almost 500,000 made, a century plus of use, and with many still being used on a regular basis. Not too many other products have been in continuous use for a century with as few failures.

5 X magnified

I agree Chuck. I have another Krag bolt with a similar crack that came from a DCM Krag that my grandfather purchased in 1928. (for $8.50) The family never shot anything but factory ammo so I know it did not crack from them shooting hot hand loads.  I have the paperwork of the ship date on the replacement bolt that was purchased somewhere, think it was 1968.  I would hazard a guess that maybe that one shot some amount of the hot stuff that was issued to increase the velocity and was stressed enough that it eventually cracked after 60 years.

Shipment Label