I added the below entry and picture to the 19th century timeline (1st half) and to the HubPages article, Backward Advances: Rear-Facing Trombones throughout History. Although rear-facing trombones were common during the 19th century (see the many examples in the HubPages article), the one pictured below is a strange version (probably not very literal in its depiction). There’s not even a brace with which to hold the slide!
1807-08—Germany: Christoph and Cornelius Suhr, in their book on military uniforms in Hamburg (Abbildung der uniformen aller in Hamburg), published in the early 1820s, depict musicians from the Dutch military in Hamburg from the years 1807-08. Included is a musician with a somewhat awkwardly-rendered rear-facing trombone (see below image; public domain).