I just added the below photograph and caption to the 19th century timeline (2nd half). The photo does not identify the trombonist, although it is well known that Arthur Pryor was a frequent soloist with Sousa, so Pryor is definitely a possibility. As a side note, it is also interesting that Sousa’s own father was a professional trombonist.

c. 1900—A photograph shows the legendary bandmaster-composer John Philip Sousa conducting a band with a trombone soloist (see image below). The photo is held by the US Library of Congress.