I pinned it down with help from a FaceBook friend. It's not a wildflower (which is why I couldn't ID it), it's a fodder plant called "Hairy Vetch" and the blossoms will become a kind of pea. It's used as animal fodder. I found several large patches, apparently growing wild, in a wildlife management area of a lake in NW Oklahoma. So it was probably planted by the staff to provide some food for the wildlife.