Tanrenbo

There is a weapon called Tanrenbo or Furibo, 

although perhaps similar to Tsukubo (the mace), actually serves as a weight, by which we strengthen the hands for later use of sword in Kenjutsu.

It could be classified as Suburito too, 

because their weight is not too heavy,

by their shape, they are still classified as Tanrenbo, 

Today they are used all sorts of things to improve grip strength, but for me personally the best is this classic, traditional and natural tools.

Most of Tanrenbo you could  found is, round, octagonal and square shapes, but I decided for the hexagon shape, for the simple reason that it is harder to make.