Added the following to the Trombone History Timeline (19th century), Alto Trombone History Timeline, and Alto in Treatises pages. Notice that, as with the vast majority of written sources, the alto trombone is not only in the key of E-flat, but is also the only trombone shown in alto clef.

1870—London, England: Adam Wirth’s trombone method, Posaunen-Schule für Alt, Tenor und Bass-Posaune…Instruction Book of the Simple and Valve-Trombone, written in both German and English, is apparently intended for a wide audience. It includes a “Table of the different Trombones” that shows alto trombone in E-flat (like the vast majority of written sources), valve and slide tenors in B-flat, and bass in F (see facing image; public domain) (Herbert, Trombone 192).