I just added the below caption and image to the Alto Trombone Timeline. What is the historical significance? This is a small piece of evidence (among a lot of other 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, 2) that the alto trombone was not used in France in particular at this time, 3) that the alto trombone was just a tenor trombone in B-flat with a smaller mouthpiece. This single piece of evidence is not conclusive; rather, it is part of a body of evidence (e.g., see the rest of the Alto Timeline, which includes more than 100 primary sources, plus here, here and here).
1888—Paris, France: François Sudre offers valve alto trombone in his catalogue. The catalogue specifies that the instrument is pitched in e-flat (see below image; public domain) (Sudre, July 1888, p. 3).