Unofficial variation (red crown with monogram) of the flag of Montenegro, with monogram of Prince Nicholas I (1860-1918); commonly used as people's flag, and also as a variation of the official civil ensign (merchant flag), that had golden crown and monogram.
Meyers Kleines Konversationslexikon. Fünfte, umgearbeitete und vermehrte Auflage. Bd. 1. Bibliographisches Institut, Leipzig und Wien 1892. (Page from the Lexikon at Commons)
original: king Nikola I; this file: Imbris