Yesterday I added the below images and entry in the 20th century trombone history timeline. The detail of the poster only shows one trombonist, and the full image is hard to see, but there are actually two trombonists included.
The creation of numerous “Damen Blasorchester” and “Ladies’ Brass Band” ensembles seems to constitute something of a small trend in the early 20th century. The IBEW website (Internet Bandsman’s Everything Within), which is an excellent brass band history source, shows 3 photographs of the Janietz Elite Damen Blas-Orchester (the group shown in the poster below). They also have photos of numerous other “Damen Blasorchester” and “Ladies’ Brass Band” ensembles from the early 20th century, many of which include trombones.
1911—Germany: A poster advertising “Janietz, Elite Damen Blas-Orchester,” a female brass band, includes 2 trombonists (see detail, far left, and full image, adjacent; public domain).