The fishing-line, hanging weight thingy, officially known as the Pro-ject anti-skate turntable weight. This is a replacement Anti-Skate weight for your turntable. Easy to lose, hard to find.
There's a reason why there is only one or two Pro-ject display turntables at Pearl Audio with the fishing line weight on it (we keep them safe in the back room), these things just DISAPPEAR without at trace. We're convinced they are sensors from a parallel universe that blip in and out of our existence after recording our conversations with Alexa.
You can wait around for a millennium or two for our galaxy to swing around to the parallel universe and the weight to re-appear or (sigh) just resign yourself to buying another. We got-em.