The Samonji sword, along with Nobunaga's armor, and a copy of Ota Gyuichi's Shincho-Ko ki are now locked up at Takeisao Shrine (Kenkun Jinja) in Kyoto.
In Thomas Conlan's great book Weapons & Fighting Techniques of the Samurai Warrior 1200-1877 A.D. (p. 116), has the engraved sword translated.
"The sword of Imagawa Yoshimoto, who was careless and killed by Nobunaga on 19.5.1560."
Last year I posted about the location on the whereabouts of the tombs of Imagawa Yoshimoto's. They are all worth a visit in my opinion. My personal favorite is the tomb at Daisyoji Temple, in Ushikubo Town.
Tenka no tame!