I just added these pictures and their caption to the Alto Trombone Timeline. For a list of somewhat earlier trade catalogues I have collected, see Alto Trombone in 19th Century Trade Catalogs.

1901—Chicago, Illinois: Lyon & Healy, a prominent American manufacturer and distributor of musical instruments, includes numerous alto trombones, all specifically labeled as E-flat instruments, in their New and Enlarged Catalogue of Band Instruments. Shown below are 4 of the altos listed in the catalogue: a slide alto and three valve altos (public domain) (Lyon & Healy 1901, pp. 45, 15, 28, 40).