One of the standout exhibits is an “odachi” long sword forged by Bishu Osafune Tomomitsu, a master swordsmith of the Bizen Osafune school in Bizen province (present-day Okayama Prefecture).

Until two years ago, the collection was known only to the man’s family members. The items include a long sword with a 70-centimeter blade inscribed with the signature of Sadatsuna, a master ...