Emp. was checking PC-6 head section drive belts, 14"" from ground. He climbed onto belt, got on his knees, leaned out over the box, he grabbed a piece of angle iron that was not connected and slipped away. He lost his balance and fell into head box (5 1/2"" deep). He reached up with his left hand & grabbed side of head box to break his fall, dislocating his left shoulder.