What is the black plastic circle on wheel axle nut? I have been seeing a black plastic part on the nuts of the wheels. My bike also has this. I think I have seen this only on old bikes. Does it have a name and purpose?



Some close up:
Those look like packing pieces used to stop the axle ends from punching through the cardboard box partially assembled bikes are delivered in.

I'd imaging they are being used to stop the axle nuts getting scuffed or scratched.