I'm not sure if they've ever come up with a conclusive answer to that one. I've seen them before. I always thought they might be used to drive a small wedge of some kind, between the stone and the shaft and hafting, to maybe tighten it up over time. Just a guess on my part and believe me when I say, "uneducated".
Heard the same pins and wedges sounds most practical, also others thoughts of showing off or lightening the weight. I have noticed the not so hardy stones don't make it that far the best or hardest stones proved with use in time often get the extra wedge treatment later on not always of course.The 2nd pic is cool can almost count the peck tap marks .Very nice axe Paleohunter.