You pocketed this rivet after taking down several Scrapyarders leaders, who went on to become the special guests of the Circle in an adventure you like to call:
Any Body Will Do
It started with a thorn in Vince Dubrowski's side: Cage Consortium's surveyors kept getting kidnapped. Vince sent you to investigate in Potter´s Field, where you learned that the Circle of Thorns was to blame for the missing surveyors. They had been capturing the men for use in their ceremonies.
Vince had to stop the surveyor shrinkage, but he wanted to do it without incurring the Circle's wrath. So he proposed an alternate source of ceremony fodder: the Scrapyarders. You entered a Scrapyarder stronghold and defeated four of their most influential leaders. This, Vince reasoned, would get the Circle´s attention.
With the Scrapyarders leaders under lock and key, Vince sent you to broker the deal with the Circle. When you entered their base, you found that the Scrapyarders had anticipated you! You assisted the Circle's mages in fighting off their invaders , and won their approval of your plan.
Now, the Circle gives Cage's surveyors leave to wonder through Potter's Field, as long as a steady supply of Scrapyarders keeps coming from Vince Dubrowski. As for you, you've earned the respect of a few Circle's mystics. Maybe that'll be enough to buy you a little protection when the time is right. Or maybe not.