I recently added a new source to both the Alto Trombone Timeline and Alto in Treatises pages. I believe that brings the total number of alto trombone primary sources to 88, as you can see in the Alto Trombone History Bibliography. Here’s the caption, below:

1897—Frankfurt, Germany: Adolph Pochhammer, writing in his Einführung in die Musik, states that the alto trombone is pitched a fourth above the tenor trombone and the bass trombone is pitched a fourth below tenor trombone. With the tenor trombone presumably in B-flat, this places the other instruments in E-flat (alto) and F (bass). Pochhammer also explains that each instrument is written for in its corresponding clef (Pochhammer 168).