I have asked two experts whom I thought might know the answer to why many cornets and some trumpets were made dual key A/Bb from around the turn of the 1900's until around 1930. Both experts responded with what seemed to me to be unlikely answers. One stated that his guess was that mutes of the time made horns so sharp that the 1/2 tone down made horns better in tune. The other person stated that A was for orchestral work and C was for popular songs played off piano sheet music therefore not needing sight transposition (Bb wasn't mentioned in that answer so it wasn't helpful). Anyone out there actually know the answer?