I have added the below caption to the Alto Trombone Timeline, where there are now well over 100 primary sources, and to the Alto in Treatises page. It provides yet another example of the alto trombone as an E-flat instrument in the 19th century. Indeed, the tallies in the Alto in Treatises page show a vast preponderance of written historical sources with the alto trombone in the E-flat orbit (rather than the B-flat orbit). For more on this subject, see also Alto Trombone in 19th Century Trade Catalogs and Extant Altos.
1888—New York: Arthur Clappé, a former faculty member of London’s Royal Military School who eventually becomes an influential bandmaster in England, Canada, and the United States, publishes his treatise, The Band Teacher’s Assistant. In his discussion of the trumpet family, he labels the alto trombone an E-flat instrument: “The trumpet family consists of a complete quartet as follows: Soprano trumpet (in almost any key), E-flat alto trombone, B-flat tenor trombone, and G, F, or E-flat bass trombone. It is to be regretted that this family is not used complete in our bands, since it imparts a richness and variety of tonal color, now deficient” (Clappé, Band Teacher’s Assistant 35).