Added the below entry to the article in HubPages called Backward Advances: Rear-Facing Trombones Throughout History, bringing the total number of visual examples to 26. The early examples like the one below are fascinating and deserve to be better known.

1732—Prague, Czech Republic: Upon restoration of the Strahov monastery, artist Siard Nosecky paints a fresco on the refectory ceiling that includes what appear to be 2 rear-facing trombones (see detail and full image below; public domain).