One of the many things I love about Steve's site is that is shows some of the number & variety of bone tools that they made and used. Most of us rarely get to find them or see them. (Occasionally a sturdy trigger awl survives, but older groups probably made a similar variety of tools to what the Mandan made.) I remember some of his famous dirt shots that showed fragments of split and cut bone littering the ground, it was a handy resource and they used it.
As for guesses go, Hairpin works because people understand what you mean, but I don't think there is any evidence to support the concept. If you look at the Mississippian and Woodland pipe and pottery effigies, I can't think of any that have hair pins (and they are oddly well known for showing hairstyles including braids, top knots, forelocks, etc.) If you look at paintings by Catlin or even some of the hide paintings by NA artists at the time, I haven't seen anything that looks like hairpins. If you look at hundreds of pictures from the mound excavations from the Ohio River valley, you see a lot of bone tools, but none stuck where the hair would be.
I'd guess it probably something entirely functional.
Most people who wore cloaks/robes, used a pin/toggle to close it. I could see something like that being used to temporarily hold two pieces of leather together. In Montana some of the groups used longer pieces of rib bone to "stitch up" the front of a tipi. Included is a picture to show what I mean. If you look at the little shelter beside the girl, something is holding together two pieces of cloth. A lot of little things are pinned to the ground. I could see a lot functional uses for something like that.
awesome find either way since they specifically designed it with that shape. Little grooves are like a few I have found but I'm also unsure of the actual use
