My sister lives in Seattle and when we take her to the airport, she usually gets on a 737-500, -300, or -700. I've seen her get on all of those. Chances are it would be the -300 or -500.
Also, SW doesn't fly direct from LAX. They stop at either Oakland, San Jose, or Salt Lake.
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.