I just added the below caption and picture of a rear-facing (or “over-the-shoulder”) trombone to the Trombone History Timeline (19th century, first half). I have also added it to the HubPages article, Backward Advances: Rear-Facing Trombones throughout History, which features more than 50 such images. As you may notice if you follow either of the above links, this permutation of the trombone is relatively common in iconography of the 19th century.

1801-50—Munich, Germany: A print depicting various musical instruments includes a rear-facing trombone (see below detail; public domain) (German National Museum, graphic collection).