Added the following 2 entries to the Trombone History Timeline: 19th century (first half). Iconography seems to suggest that the rear-facing trombone was quite common in the early 19th century. It’s noteworthy, also, that they’re both depictions of dances (see Trombone and The Dance, part 1 and part 2 for more on this).
c. 1820—France: An etching titled The French Garrison, probably set in Normandy, depicts a group of French soldiers mingling with villagers. A fiddler and a regiment trombonist with a rear-facing instrument provide music for dancing while standing on a makeshift stage (see below detail and full image; public domain) (Fromrich 24).
1828—Paris, France: Caricaturist J. J. Grandville depicts a country dance in his lithograph, Sundays of a Good [Middle-Class] Citizen. At the front-center of the orchestra is a rear-facing trombone (see below detail and full image; public domain) (Fromrich 29).