Self Locking Acorn nut
 
Acorn nuts are the finishing touch on 70s motorcycles.  ・It is made of two parts, a hexagonal nut and a friction ring (special spring) of general JIS standard. It is a nut with a stable anti-loosening effect that is integrated by fixing the friction ring by caulking on the upper surface of the nut.
 
Japanese motorcycles usually use fine pitches for M10 and above. So you may need M10 x 1.25 and M12 x 1.25 which are not common in hardware stores and overpriced in bike shops.