B744 built with winglets; B777 not due to more efficient wing design as mentioned.
B737 Classics not originally designed with winglets but 737-NG's were thus the difference you observed on your SWA flight. I believe SWA are retrofitting their classics though as I'm certain I flew on a 733 recently with winglets. BA did same with B757 for the Openskies operation as have United/Continental, US Airways and others.