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.