Would that be Air Southwest based in UK with 5 Dash 8 -300s, or Southwest Airlines based in USA with 544 737s?
Performance-enhancing Blended Winglets have been added to our fleet of 737-700s, and all new 737-700 aircraft arrive from Boeing with Blended Winglets installed. Additionally, Southwest began installation of Blended Winglets on some of our 737-300 aircraft in early 2007; installation of Blended Winglets on these aircraft was completed as of the end of second quarter 2010.
Southwest Airlines Factsheet