Black Ratchet Wingnut with M5(5mm) thread, the length of screw is 18mm. It is compatible with railblock 1266 and railblock 1406.
It is spring loaded, and can be pulled out and turned to a more user friendly position after being tightened.
It looks simple but it is very useful in some scenarios. It comes with a M5(5mm) screw on the top and a wingnut at the bottom. It could be mounted on accessories with M5 threaded hole. Great!