When I have flown international on Southwest it was possible to obtain a boarding position 24 hours prior to departure without entering any passport information, it could be added later, anytime before the flight. Even if you enter incorrect information and you are "flagged" you should still have your boarding position. Did you check in 24 hours prior and not get a boarding position? This seems very unusual. You are correct that you will not get your actual boarding pass until you get the check-in counter, but that is different than getting your boarding position. --TheMiddleSeat
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