It is a black nylon/metal nut, therefore bi-material, with a diameter of 30 mm. This is the BPF 150 model. This wing nut with 12 mm diameter fins. It is a type F nut, therefore non-opening (closed).
Thanks to its butterfly shape, you won't need any tools to screw it on. It screws on easily by hand.
In addition, it has increased strength thanks to its bi-material side.
These wing nuts are often used in construction, industry and furniture.