A first-year Stanford medical student has made a very special Father's Day present for his dad--a state-of-the-art shoulder joint and arm to replace the heavy plastic prosthesis his father has worn for 29 years. Doug Parker, 23, designed the arm for his father, Clarence (Dude) Parker, as part of a research project at Children's Hospital on campus. The senior Parker, 68, a rancher who also plays mandolin in country bars, lost his arm and shoulder in a hay baler accident. "It sure feels good," he said from his home in Bridger, Mont. The new prosthesis weighs about three pounds and replaces a cumbersome, six-pound device that no longer fit properly and caused some pain.