I recently added 3 entries to the 19th Century Timeline (1st half, 2nd half) and Alto Trombone History Timeline from 19th-Century American newspapers. All three entries deal with the alto trombone. Two are from advertisements, the other a report of a fire. In terms of key, the advertisement from Dodworth of New York actually specifies “alto trombone in E-flat.”
1843—Hudson, Ohio: An advertisement in the Ohio Observer labeled simply “Boston,” (though offering a Cleveland street address) mentions numerous musical instruments for sale, including “Alto & Tenor Trombone” (Ohio Observer).
1852—Milwaukee, Wisconsin: Among a number of instruments lost to members of the Milwaukee Musical Society in a fire is a “trombone alto” (Milwaukee Daily Sentinel).
1882—Raleigh, North Carolina: Dodworth of New York advertises several instruments for sale, including an alto trombone in E-flat (Raleigh News and Observer).