This wakizashi (companion short sword) has an inscription of a noted swordsmith, Sadamune. It is indeed an excellent sword, but the authenticity of the inscription is not clear. This is because the inscription was engraved after the long sword was made into the short sword by cutting the tang where the original name was inscribed.
According to a document of the Tachibana family, the first lord of the Yanagawa domain, Tachibana Muneshige, was equipped with the following three swords in the Invasion of Korea: ken sword ‘Nagamitsu’, katana sword ‘Kanemitsu’ and this wakizashi sword ‘Sadamune’.