Originally Posted by fietsbob

that looks like a Plastic pop-rivet.. in the picture ..

I used a Stainless Poprivet for the mud flap replacement , after my Brompton lost it's screwed on one ..

Of course you can melt plastic and then they wont unscrew .. touch the tip of a hot soldering iron on it.

..then again , a stainless bolt and Nylock nut is a reliable combination..