Don't know about their sousaphones specifically, but "Stratford" was their lower end or student line of instruments. If it's not a BBb, then it's an Eb. I doubt very much that they made any sousaphones in F.
I have played exactly one Besson sousaphone and it was in BBb, but I wasn't impressed. It was stuffy and sounded thin. Their sousaphones do not generally have a very good reputation for sound, intonation, or playability.