For authenticity on this question, I'd listen to B Shipman.
Probably not many people looking at your fowler will know, or give a hoot what type of nail is holding the thumb piece, but you will, and you're the one that counts.
So study pics of NE fowlers and use the appropriate material and head style, and also note the positioning of the nail, or nails on the originals.
I'd answer the question if I knew, but I haven't studied NE guns much.
John