Norton Nut for eye bolt, line 4 (3/8" British Standard Cycle)

This is the nut that pairs up with an eye bolt on the frame side, sized for 3/8 inch British Standard Cycle threading. It's a small hardware item but it shows up in a handful of different spots on the bike - the catalog record shows it across frame assembly, the handrail and support rail, the rear carrier and safety bar setup, the oil tank and sidecover mounting, and the footrest and brake linkage areas. That spread tells you this is a general-purpose eye bolt nut Norton used wherever an eye bolt needed securing, not a single dedicated fastener for one bracket.