I recently added this caption (below) to the Alto Trombone Timeline, bringing the total number of primary sources in that timeline to 93. The location is noteworthy; there are not many sources from Spain on the alto trombone.

1899—Madrid, Spain: Luisa Lacál, discussing the trombone family in her lexicon, Diccionario de la música, téchico, histórico, bio-bibliográfico, states that the alto trombone (trombone contralto or alt-posaune) is pitched in E-flat (Lacál 549).