I know that the pilots that lurk here don’t work for Southwest, but I still might give this question a try.
Having only 1 type of aircraft (B737) and a few variants (300, 700, 800) is it the most senior pilots that fly the 800 aircraft and the newer recruits start off on the 300 or 700? In shorter terms, I know pilots bid for the TYPE of aircraft they fly, but do they bid for the VARIANT?
While I don’t fly for SWA I do have friends who do and the answer is no, at least not directly. Since SWA only flies 737 there’s no other aircraft to bid. Pilots bid for days on/off and overnights or routes. Now since SWA may put a specific variant on a specific route that may (I suppose) cause a pilot to bid a particular route or trip to fly that plane.
Btw, When I asked my friend he asked me to ask “why would we?”.