Southwest has utilized 727-200s on two separate occasions:
N406BN 291 19991/521 N7276F 03/79 01/80
This was a "Basic" 727-200 leased from Braniff (v1.0) and it was related to a court settlement at the time. Southwest wanted to try another sized aircraft to see how much it might it might impact the 10-minute turn that was the standard (for the 118-seat 737-200s) at the time. The 727-200 was configured to 155 seats, and the turn times took longer. Personally, I think they did this as a test in anticipation of the 737-300s, which were under development around this time.
Southwest went ahead and ordered the 737-300 (137-seats), and the first ones started showing up in November 1984. The following "ADVanced" 727-200s were all leased frrom PeoplExpress, and were used for 12-24 months to provide additional capacity until a sufficient number of new 737-300s could be delivered. The color scheme on these were a little bit different than N406BN, but they also held 155 seats.
N551PE 227(A) 20772/982 551 N426BN 02/84 02/85
N561PE 227(A) 21043/1113 561 N436BN 08/83 08/85
N563PE 227(A) 21045/1133 563 N438BN 05/84 10/85
N564PE 227(A) 21118/1167 564 N439BN 08/83 08/85
N566PE 227(A) 21242/1196 566 N441BN 05/84 10/85
N569PE 227(A) 21245/1202 569 N444BN 05/84 09/85
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.