Joe, The knob is a stop that keeps the drawer from coming out to far and fall. The grove is a glide for a strip of wood glued to the side of the inside cabinet. It is drawer guide. Your cabinet did not have the guide, the drawer rides on the rail as in the right side of the inside cabinet. this type of drawer side was mass produced so the builder could us several methods to to create a drawer. Youre drawer will be like the pic I posted of the center of the cabinet where the drawer slides into. Boy you have a lot gluing to do on this one but should be within you talents.