Who? Where? When? II

Kudos to Chakira for correctly identifying the Mystery Rabbi as R. Marcus (Mordechai) Horovitz, one of R. Esriel Hildesheimer's top two students (the other being R. David Zvi Hoffmann), son-in-law of R. Ya'akov Ettlinger (the Arukh La-Ner), rav of the Gemeinde Orthodox community in Frankfurt Au Main (and hence R. Samson Raphael Hirsch's archenemy), close friend of Orthodox historian R. Yitzhak Isaac Halevi (author of Doros Ha-Rishonim) and author of Responsa Matteh Levi.

Judging from how he looks in the picture, I'd guess he was around 50 years old at that time which means it was taken in the 1890s.

UPDATE: Greg found this bio and this picture.