Among the most rare Okinawan studies I have found is that of the Surujin. A flexible weapon of a rope and stone in its original version. Versions with metal and chain in later incarnations.
Of course in those systems which teach this, it would not be rare, but to other disciplines..
From ‘Kama, Tekko, Tinbe and Surujin – Ancient Martial Arts of Ryukyu’ by Inoue Motokatsu. “Surujin is the best weapon for hiding the opponents eyes but it can't be used without the right how to use and enough training.”
Being that is is a flexible weapon, it is akin to arts from China, being the Steel Whip or the weighted rope dart weapons.
From the Innoue School
From the Matayoshi School
There is a real chance of self injury when training if due diligence is not used.