Conmetall Nylon Insert Nut M5, A2 Stainless Steel.
Nylon insert lock nuts have a nylon collar that grips the bolt thread and resists loosening under vibration. Used on machinery, trailers, gates and any joint that moves or vibrates in service.
Tighten with a spanner or socket, and use a new nut when reassembling as the insert loses grip once removed. Supplied in a barcoded Conmetall retail prepack.