A good friend of mine in Iowa, that used to live here in the Bitteroot Valley has a piece of Bison Skull found somewhere around here, with a point very similar to that one, has been verified as an atlatl point and is prehistoric. I still think your best bet is having a Zoology or anthropology professor and some University look at it, We can sit here and speculate all day long and nothing positive will come of it. The University of North Dakota I understand has a great department and might be able to give you some idea from good photographs. You might even email them to a number of different schools and get a bunch of opinions.
Bob