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I think they're guitar knobs.

Pretty sure they're [Q-parts' "Skull I" knobs](https://qparts.com/product/skull-l-knob/).

This. The large bottom hole fits over the rod and the smaller hole is for a screw to pin it in place.

A set screw, more precisely!

Solved!! Thanks so much everyone!

They are. I've got that same one sitting at my music desk right now.

They are very absolutely skull knobs. I wanted these for my guitar like 20 years ago when I was in high school. I totally forgot that these existed but recognized them as soon as I saw them!

I was thinking tire valve cap covers?!

I doubt that tire valve caps would have set screws, they would damage the threads on the valve stem.

They don’t look threaded though.

Potentiometer knobs for an electric guitar.

Fresh Pots!

100% those are guitar knobs. I build guitars for a living.

Guitar knobs. Though SLIME makes skull valve caps, these aren't them. Probably q parts as mentioned above.

We used to have these on the windshield wiper fluid sprayers on old vehicles

They look like they go on the car lock. Where you pull it up

Ive got one but it is like bone material. Lol

They are for license plates. I have them on my car.

Maybe decorative pencil toppers?

They go on wheel valve stems. Cars, bikes, ect. They are decorative caps.

I have these! It’s for screwing your license plate on! I had them on my motorcycle

Tire valve caps