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!