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.

1616—Bologna, Italy: Ludovico Carracci’s Paradise, an altarpiece painting located in the Church of San Paolo Maggiore, features an angel-trombonist situated prominently among a group of angel-musicians (see below detail and full image; public domain) (Komma 109; Emiliana 167).

1606—Innsbruck, Austria: Paolo Piazza’s Adorazione dei Magi, an altarpiece painting in Innsbruck’s Kapuzinerkirche, depicts many angel musicians, including an angel trombonist (see upper-left of image below; public domain) (Panchieri 43).

Peter Spring altarpiece, Fribourg

1592-1601—Fribourg, Switzerland: The Augustinian monastery’s altarpiece by Peter Spring depicts a group of angel-musicians, including an angel playing a trombone (see above image; public domain) (Wold 82).

Detail, The Engagement of St. Ursula and Prince Etherius

The Engagement of St. Ursula and Prince Etherius

c. 1520—Spain: The Engagement of St. Ursula and Prince Etherius, sometimes also titled St. Ursula and Prince Etherius Making a Solemn Vow, a panel painting from the Master of Saint Auta Altarpiece, depicts a trombonist with a wind band performing from a loft or window. The painting may represent the earliest visual depiction of a black trombonist (see above detail and full image; public domain) (Lisbon, Museu Nacional de Arte Antiga, Inv. No. 597) (Bowles, Musikleben 30-31).

Detail, Hans Baldung, Coronation of the Virgin

Hans Baldung, Coronation of the Virgin

1516—Freiburg, Germany: Hans Baldung’s painting, Coronation of the Virgin, the central panel of an altarpiece located in the Freiburg Cathedral, includes an angel-trombonist among a group of angels playing wind instruments above and to the left of the Virgin (see above detail and full image; public domain) (Burkhard pl. 2).

Detail, Elsheimer, The Exaltation of the Cross

c. 1595—Frankfurt, Germany: Adam Elsheimer’s, The Exaltation of the Cross, part of an altarpiece of several copper panels, portrays an angel playing trombone among a group of other angel musicians. Elsheimer, known for his variety of light effects, places the trombonist near the burst of light at the top of the painting (see above detail; public domain: wikimedia commons) (Klessmann).

Lastmann, David in the Temple

1618—Pieter Lastmann’s altar scene, David in the Temple, portrays a trombone performing with voices, tamborine, pommer, and 2 string instruments (see above image; public domain) (Kinsky 177; Buchner 254).

Bueno and Cano, Virgin of the Rosary altarpiece

c. 1620-24—Seville, Spain: The altarpiece of the Virgin of the Rosary of the Parish of Santa Ana, probably painted by Diego López Bueno and Miguel Cano, includes a depiction of an angel playing trombone (see bottom-right of above image; public domain image) (Alonso Cano 613).

1516—Freiburg, Germany: Hans Baldung’s painting, Coronation of the Virgin, the central panel of an altarpiece located in the Freiburg Cathedral, includes an angel-trombonist among a group of angels playing wind instruments above and to the left of the Virgin (see facing detail and full image, below; public domain) (Burkhard pl. 2).