Those are short pieces of welding rod. A wire coat hanger would serve the same purpose.
Costco installed rubber stems on the first set, so I wanted to ensure the hook style extender wouldn't cause issues, which is why I created & installed the wire clips.
When those Michelins were recalled, Costco installed new stems, but this time they were metal, but still with a rubber base. A much more substantial base, but still a rubber base. I re-installed the clips.
I've yet to look at the stems on the new Sunseeker, but from what I've read, the wheels are supposed to have all-metal stems. (with a gasket/o-ring, of course) I'll install the same type of extenders on the Sunseeker before the winter is out.
On the ford, I initially tightened the inner fitting with a tiny set of channel-locks on the ford rims - it was a major pain. Subsequently, I installed the extender after balances, before the wheels were re-installed on the rig. I learned that it was easier to remove & haul them to Costco in my pickup rather than have them deal with it.... parking lot with jacks, anyone?
I posted an article on the recall, but can't seem to find it at the moment. Here's some images:
Michelin Tire Recall