I just added the below caption to the Alto Trombone History Timeline. A link is provided to the museum holding, so you can have a look at the alto trombone. The instrument is pitched in E-flat and is clearly not a small-bore tenor trombone.

c. 1879—London, England: Besson’s London factory produces an alto trombone in E-flat with “Medals of Honour” stamped on the instrument. A Besson stock book, 1887-1895, indicates production and sales of alto trombone, and a Besson advertising poster from c. 1885 features an E-flat alto trombone, labeled, “Slide Trombone, in E-flat (alto),” as well as a valve alto trombone, labeled, “Valve Trombone, in F or E-flat” (Horniman Museum, London; Myers and Eldredge, Brasswind Production).