It is a very strong 20 mm diameter knurled nut, thanks to its nylon head and zinc-plated steel insert. This is the BPF 105 model.
The knurled nut allows manual assembly, is very resistant and is anti-corrosive. It does not conduct electricity.
You can screw in by hand, and thus assemble your inaccessible parts with a tool.
They are often used in aeronautics, industry and the electrical field ...