Tie tack?

[Detector mom]

Found in backyard, where I?ve found lots of early wheaties, that was once a farm field in heavy use in the mid-1800s. It appears to be copper.

 

[crashbandicoot]

Roofing nail,most often used to nail on the underlayment before the shingles go on.

 

[Mackaydon]

I remember another member found the same thing; may have something to do with roofing; like a rivet for shingles.
Don.....

 

[Red-Coat]

Over here we call 'em copper "tail rivets" and they're generally used for securing roofing tiles or slates. These days the heads are usually round, but older ones are often octagonal. They're secured as shown in the diagram.

 

[bowwinkles]

When I was a kid we called them "tar paper nails".