The Nanban Mirror is an ancient artifact that had been with Cologne's family for many generations until it was stolen by Happosai.
Its only appearances are in the anime-only arcs Back to the Happosai! and Ranma Goes Back to Jusenkyo At Last.
This enchanted mirror can be activated having a tear cast on its face. When that happens, it will transport the bearer and anyone nearby to a spot in space and time chosen by the bearer.
In its first appearance, Happosai accidentally transports himself, Cologne, Soun, Genma, Akane, Ranma and Shampoo to the Joketsuzoku village in China some three hundred years ago, when Happosai and Cologne were both teens.
In its second appearance, Ranma, Genma and Happosai first travel through time several years, manifesting in a Tendo Dojo where a miserable old Genma and Happosai are living with Akane, her husband Ryoga, and their quadruplet sons. They then travel to China, just outside Jusenkyo, on the day that Ranma and Genma are due to arrive there for their fateful curses. They then travel back to the Tendo Dojo, though they somehow "misaim" themselves and end up appearing on the day that they would have originally arrived there.