According to the Anime (and Manga) Rurouni Kenshin, there's a mythical sword by the name of "sakabato" out there in the wild, one made only for the warrior most dedicated to pacifism. With the blade cut into the back side of the curve in this katana rather than the front, it's only as a final resort that the warrior takes to cutting their opponent. A real-life version of a sword just like this has been discovered in a family storage cellar in Japan this week.
While it's impossible that the writers of Rurouni Kenshin knew of this unique sword's existence specifically, there have been bits and pieces of knowledge of such weapons having been used in Japan's long history. It's through Asahi News that we hear of the tiny Shirai sword here, and through Jidai (sword shop) that we're to understand that the Deoksu Royal Palace Museum in Korea holds several examples of a rare reverse blade that could possibly have been the inspiration for the reverse blade we're seeing here, planted firmly in Japan's history.
The discovery this week is being analyzed now, with the best technology available to authorities in the area aging the weapon back to Japan's Edo Period (1603 to 1868). Work still must be done to source the materials that went into the blade itself - there's always that chance that the weapon was simply shipped from Korea for whatever odd reason - though such a shipment would also be historically significant.
The weapon is said to be engraved with dragons and rusted from top to bottom, making it difficult - at first - to date the underlying metal makeup. The blade has been donated by the family that discovered it to their prefecture's cultural archive - we must assume they will continue to study its makeup there. Hopefully this isn't the last we'll hear of this tiny blade!