Hello Tim,

I know of a roofing nail (for corrugated iron roofing) that was patented
around 1887 (British I think) with a lead head, shaped a little like a hat,
but with a drawn-wire iron shaft. I can provide the reference if needed.
Considering the shaft was like a cut nail and the head diamond shaped this
is perhaps of little value. However, the combination of lead with iron may
help to narrow the date of your nails.

Regards,   Robert

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