Yamamoto's Bat is a bat that was given to Yamamoto by Reborn shortly before the Kokuyo Arc.
Before receiving the Shigure Kintoki or learning the Shigure Soen Ryu, Yamamoto used a bat that, when swung at 300 km per hour, transforms into a blade. His first bat broke during his fight with Ken Joshima but Reborn gave him a second one, exactly the same.
The bat was later replaced by the Shigure Kintoki, given to Yamamoto by his father, that transforms from a bamboo sword into an actual blade in a similar way.