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?