These attentive bleacherites are scouts for 13 major league clubs. They assembled in Pittsburgh the other day to look over a left-handed young man named Sam McDowell (right). A 17-year-old senior at Central Catholic High School, Sam has pitched 40 no-hitters over a nine-year baseball career. According to the rules, of course, the scouts can't do anything about Sam until he's out of school. But graduation day is this Sunday, and guess who's going to be waiting for Sam when he gets home with his sheepskin ?