What are these things to?

[Kapidr]

From time to time I find these while detecting Florida beaches. What are they? Any help would be appreciated. Thanks.

 

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[Back-of-the-boat]

Rivets.From what I don't know.

 

[Nitric]

This is a way out there guess! Could they be from folding lounge chairs? I remember scraping a few, years ago and taking the rivets out of where the chairs folded. I don't remember exactly what they looked like or what they were made out of. But I remember they were about that long with a washer.

 

[cw0909]

i think a good Id Nitric
aluminium folding lounge chairs, you can see it/rivet in the
frame of the chair in foreground

https://img0.etsystatic.com/000/0/5129390/il_fullxfull.341657262.jpg