The story recounted to me, from a source I regard as reliable, is this:
Szell was on a guest engagement with Vienna and became enamored with the sound of their trombone section. Prior to returning to the United States, he convinced the Vienna section to sell their instruments to him en masse, and presented them to the Cleveland section as their new tools of trade. Apparently, these instruments were not in the best repair and their playing characteristics might have contributed to the Viennese' eagerness to part with them. The Cleveland players compromised with Szell on condition that they could locate and acquire domestically produced instruments that played better and still produced something of the Viennese sound Szell desired. Working with the craftsmen at the King plant, they came up with a variant of the 4BF with a larger bell, and thus was born the modern version of the 5B (a .547" tenor with what would later be the Conn "K" bell), replacing the previous 5B which was a .536 "bass" trombone.
