A knuckle is a joint, like a knee or a hip. I think the neck with a front knee bone works the best. After you roast it for color, deglase the pan and boil with mirepoix, and your sachet. Start with cold water and simmer away. You could use any veal bone for flavor but the collegen comes the joints. I have used beef bones for stock but you cannot get demi from the older bones.
I hope this helps, this comes right from my "On Cooking book".