Basic brown plastic tsuba, a guard that will protect your hands during full contact practice, which is fixed on your bokken with a rubber ring.
This plastic Tsuba is the only made in Japan plastic Tsuba designed for classic Bokken on the market and only exists in brown color.

It will fit on all standard and high-end bokken, with or without groove, and on chuto (short bokken) and shoto (wakizashi), but might require a little filing depending on the exact shape of the blade (particularly at the mine).
Please make sure you select the correct size as the hole is slightly smaller for Shoto size. The M (Chuto) size can still be recut for a Bokken, but a Bokken Tsuba will be too large for a Chuto.