From the 1959 King Catalogue:
The symphony Model Trombone is a long time favorite among outstanding artists and dance trombonists. Its special large bore and 9" bell produce a study masculine tone quality that is a prime requisite for large musical organizations. The upper register, too, has a clarity and evenness of tone that is unusual for a large bore instrument of this type. Now equipped with a slide lock.
Gendered tone quality not withstanding, this is a monster trombone that can easily shift from Brahms to Big Band to Bobplicity. You would be hard pressed to find a more flexible trombone.