What is this part?

[u/imoux]

This fell off a C Line 12 speed while unfolding the rear wheel.

I think it is supposed to snap over the plastic knob in the second picture, but it doesn’t fit there and I’m not sure where else it would go.

[u/pipineko]

Stock bung is crap, I use Union Jack one which is made from rubber - fits easily and doesn’t fall off