✅ SOLVED Any idea

[mmocha2905]

Any idea

 

[Red-Coat]

I think they're what we call 'tail rivets" on this side of the pond and they're generally used for securing roofing tiles or slates. They're usually copper and these days the heads are usually round, but older ones are often octagonal. They're secured as shown below:

 

[mmocha2905]

Thanks!