Because their weight is not so heavy (2,5 Kg),
it could be classified as Suburito too,
but 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, also shape of tsuka os not round as you could see on last photo.