It is said that Muramasa moved from Mino to Kuwana and lived there between 1467 to 1469, and the oldest sword in Japan was found in 1501.
Kuwana has great facilities for water-transport for a long time, and Muramasa's sword were also handled down places to places, and his swords were highly evaluated and loved by samurais in Mikawa district.
However, when the father of Tokugawa Ieyasu was killed, and when Tokugawa Nobuyasu (Ieyasu's son) was forced to kill himself by Oda Nobunaga, in both cases the swords were Muramasa's work. Therefore Ieyasu hated Muramasa's swords.
When Ieyasu became master of the Empire, feudal lord removed Muramasa's swords for Ieyasu.
Hence, the name of Muramasa was used for fourth, and later than that, they changed the name to "Sengo".
Kuwana City Museum