Well, if you ask me, the 2nd phalanx from a horse is symmetrical if cut down the middle, because a horse has one hoof. In bison, the joint end of the bone is curved inward, because there are two hooves. Also, a bison bone would typically be larger, from my experience.
From your pictures, it looks too symmetrical for bison.
That being said, I hope Harry chimes in, I'd like to know his opinion.
Here is one of mine for comparison.