JasonTX, the pictures you sent are blurry and too close up, but I am guessing that they are smooth nubbed, not grooved like an axle shaft spline.
I just bought a wiper arm from WH, and with it came the splined nut adapter(in case my bronco didn't have it) you can attach it with allen head set screws that come with it.
Not sure if they sell seperate from the wiper arms.