I also added the image to the Ophicleide History and Images post, which I continue to update from time to time.

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c. 1830—Vienna, Austria: K.k. Österreichischer Militair Leichen-Conduct, lithograph no. 8 from a series edited by Michael Tretsentsky, shows a military band that includes trombones with both traditional and rear-facing bells (see below image–click image for larger version; public domain) (Pirker).

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).

I am not going to try to come to any real conclusions with this post, but I would suggest the early rear-facing trombone as one possibility. There are at least two iconographical examples of rear-facing trombones from the 17th century or earlier (see here), as well as many others from the 19th century (see here). In the case of the first three examples below with no apparent bells, the bell on a rear-facing instrument could easily be obscured by the player’s head and/or other parts of the image, such as cap, halo, and wings. In the final example, from the 19th century, the bell is very likely obscured by the player’s helmet; there are multiple other examples of rear-facing trombones in iconography of this precise period and region.

c. 1474—Asciano, Italy: Matteo di Giovanni’s The Assumption of the Virgin, the center panel of an altarpiece in S. Agostino, includes what may be an angel-trombonist along with several other angel-musicians. The instrument has what appears to be a slide but no visible bell (see below detail and full image; public domain) (Belán 111).

1503-1529—Spain: Joan Gascó’s painting, God the Father and the nine angelic choirs, includes what appears to be a trombone-playing angel, grouped with 2 other wind-playing angels, although no bell is apparent on the trombone (see below detail and full image below that; public domain) (Ballester).

1598-1606—Valencia, Spain: Bartolomé Matarana paints a fresco of angel musicians in the the church of Real Colegio–Seminario de Corpus Christi that includes what are probably 2 trombones. Only the slide portions are obvious, although possible bell flares can be seen upon close inspection (see details and full image below) (Olson, Angel Musicians).

c. 1850—Brussels, Belgium: Musicien et trompette de cuirassiers, a lithograph by Henri Hendrickx, portrays a Belgian infantry musician playing on what may be a rear-facing trombone, the bell obscured by the player’s helmet (see below image; public domain) (Bibliotheque royale Albert I; Wangermée vol. 2, 263). See other Belgian rear-facing trombones from the same time period at this post: Belgian Military Trombonists.

1526—In the series of woodcuts titled The Triumph of Maximilian, executed primarily by artist Hans Burgkmair, 2 images (Plates 77 and 78) include trombones on horseback. The instructions for the engravings read, “After them shall come on horseback Burgundian fifers in the Burgundian colors with bombardons, shawms, and rauschpfeiffen. And they shall all be wearing laurel wreaths.” See above 2 images (Burgkmair, Triumph 9, plates 77-78; public domain).

1825—Belgium: Sir George Smart, a British traveler, observes, “We heard a very good military horse band which was all trumpets, bugles and tromboni” (Smart 66).

c. 1829—Mainz, Germany: Artist Joseph Scholz depicts a group of 4 military musicians of the Prussian Army on horseback in an image titled Preussisches Heer–Garde Artillerie (see above detail of trombonist; public domain) (Ryan, Paper Soldiers).

c. 1850—Brussels, Belgium: Musicien et trompette de cuirassiers, a lithograph by Henri Hendrickx, portrays a Belgian infantry musician playing on what may be a rear-facing trombone, the bell obscured by the player’s helmet (see above image; public domain) (Bibliotheque royale Albert I; Wangermée vol. 2, 263).

c. 1885—Scotland: Military musicians from the The Gordon Highlanders, a British army infantry regiment from 1881 to 1994, are depicted in an image subtitled Drummer and Bandsman. The regiment takes its name from the Clan Gordon and recruits largely from Scotland (see facing image; public domain).

c. 1795—France: Trombonist André Braun publishes Gamme et Méthode pour let Trombones, the first complete modern method book written specifically for trombone. It also contains the first description of a tenor trombone in B-flat with seven chromatic positions (see below image; public domain). It specifically states that the alto trombone is in E-flat, the tenor and bass in B-flat. An influential method book–it informs several later methods, including Fröhlich and Nemetz–the method book also provides an alto trombone position chart clearly showing an instrument in E-flat (see Alto Trombone History Timeline). Editions of the method are subsequently published in German and Italian (Weiner, Braun; Weiner, Braun Revisited; Dudgeon 194).

1847—Paris, France: An article in the periodical L’Illustration titled “Le Pupitre de Palestrina” (“The Desk of Palestrina”) is accompanied by an image by Henri Valentin that includes a rear-facing trombone in what appears to be an attempt at a historical representation of a Palestrina-era performance (see far left of below image; public domain) (L’Illustration, vol. 9, May 1, 1847, p. 137).

1853—Paris, France: The illustrated newspaper L’Illustration publishes a graphic, “La danse aux camps,” depicting a military celebration with a four-man band in the upper-left that includes what appears to be a rear-facing trombone (see below image; public domain) (L’Illustration, vol. 22, July 23, 1853, p. 64).

1804-1815—France: A military illustration labeled French Napoleonic Band depicts the foot grenadiers of the 1st Regimental Imperial Army Old Guard, including two trombonists marching with buccin-style instruments (trombones with dragon-head bells) (see above detail and full image; public domain) (Cassin-Scott and Fabb 15).

c. 1839—Fribourg, Switzerland: Musica militaris, a print depicting musicians of the boarding school of Fribourg, features a percussionist and a trombonist with a rear-facing bell (see above image; public domain) (Bovet and Curchod 28). For a similar image from Belgium, see 1831, above.

1840—France: A print depicting a Napoleonic military band includes what could be a buccin-style trombone (dragon-head bell) (see far left of above image; public domain) (Brenet 125). For a similar image, see 1804-1815, above.

1908—England: Military illustrator John McNeil (b. 1872) paints numerous military watercolors for Gale & Polden, including two that feature trombone: Highland Light Infantry (see upper image; Harrington 10) and Worcester Regiment (see above lower; Harrington 39) (images public domain).

1824—Belgium: A pen and ink drawing of a Belgian military trombonist depicts a player in full military regalia with a rear-facing trombone (see below image; public domain) (New York Public Library Digital Gallery).

1825—Belgium: Sir George Smart, a British traveler, observes, “We heard a very good military horse band which was all trumpets, bugles and tromboni” (Smart 66).

1831—Belgium: A lithograph titled Musiciens d’infanterie belge, or “Belgian infantry musicians,” portrays a musician holding what appears to be a rear-facing trombone (see below image; public domain) (Bibliotheque royale Albert I; Wangermée vol. 2, 264).

1832—Belgium: A print by Belgian artist Jean-Baptiste Madou titled Officier Garde Civique Premier depicts an officer of the Belgian Civil Guard. Behind the officer stands a military musician who appears to be holding a rear-facing trombone (see below image; public domain) (New York Public Library Digital Gallery). For a similar image, see 1831 (above).

c. 1850—Brussels, Belgium: Musicien et trompette de cuirassiers, a lithograph by Henri Hendrickx, portrays a Belgian infantry musician playing on what may be a rear-facing trombone, the bell obscured by the player’s helmet (see below image; public domain) (Bibliotheque royale Albert I; Wangermée vol. 2, 263).