Frank:
Maybe two men/two handguns. One large caliber-low velocity (cowboy loads), the other smaller caliber....possibly derringer.
First shot from derringer left him mortally wounded but still moving, second shot from revolver finished him off. Neither shot may have had the velocity to exit through any thicker part of the skull, or the larger may have exited through the palate or an eye socket.
Or....possibly a pick was used, with the second and heavier blow used to finish him off.
It would take a heluva tumble to put two holes in a skull like that.
No confessors. So it's all speculation IMHO.
Mountain Lions go for the neck-spinal cord. Bears can go for the head or neck, or even a leg.....shake em up like a dog and drag them off. Death usually follows quickly from shock and blood loss.
I offered a suggestion a long time ago, that if Ruth's body WAS moved post-mortem, and this was done a few days later, that decomposition and/ or animal predation had left the skull barely attached to the rest of the corpse. If the body had been wrapped in a blanket and draped over a pack horse, I thought it possible that the head may have been "lost" (detached from the corpse) somewhere between the murder site and where the body was recovered. Whoever moved the body wasn't about to backtrack and search for the skull, or if wrapped in a blanket, may not have noticed.
Regards:Wayne