The easiest way to tell a Boeing 737-500 from a Boeing 737-700 series is by looking at the flap actuators under the wing. Southwest makes it easy to spot because on their planes, the actuators are painted red on the 700 series planes and they are white or body color on all the other series planes.
The flap actuators exist to help the flaps lower and raise and the covers are there mostly for aerodynamic reasons since their shape helps reduce the drag that the exposed flap actuators would create.
In reality though, nowadays (mid-2020) it’s rare to see a 737-500 in Southwest colors anyway. I feel that it’s still a neat little detail.