I just added the below caption to the Alto Trombone Timeline. This addition brings the total number of 19th century trade catalogs that clearly label the alto trombone an E-flat instrument to 22 (see here). What is the historical significance? This is yet another piece of evidence that runs counter to what some historians have asserted about the alto trombone: 1) That alto trombone was not used (or was rarely used) in the mid-to-late 19th century, and 2) that the alto trombone was just a tenor trombone in B-flat with a smaller mouthpiece. This new piece of evidence is not conclusive by itself; rather, it is part of a large body of evidence (e.g., see the rest of the Alto Timeline, which includes more than 100 primary sources, plus Alto Trombone in 19th Century Trade Catalogs, Alto in Treatises, and Extant Altos). See also this noteworthy recently-published letter by Johannes Brahms from the year 1859, wherein Brahms advocates very strongly for a “genuine little alto trombone.”
1884—Boston, Massachusetts: John C. Haynes & Co. publishes its Illustrated Catalogue of Musical Instruments. Included among the trombone holdings is an alto trombone, specified as an E-flat instrument (see below image; public domain) (Haynes 1884, p. 43).