At left, a genuine 19th century coffin nail found at Fairview Cemetery in Santa Fe, NM. At right, a nail found in Greyfriars Kirkyard in Edinburgh, Scotland -- probably 18th or 19th century.
While I'm quite certain that the Fairview nail is a genuine coffin nail (it having been found amongst human bones, coffin wood and other coffin hardware in the graveyard) I am less sure if the Greyfriars nail is actually from a coffin, or if it was left over from some long-ago construction work done in the area. It also could have been from a wooden cross or wooden gravemarker that's since disintegrated. However, human bones are known to work their way up to the surface from the heavy rains in the area, so it's possible that it could really be one that found its way out... nevertheless, only the Fairview nails are offered on my Coffin Nails for Sale page, since those are the only ones I'm sure about.
These kind of flat-sided nails were handmade or partially handmade. Until machine-cut wire nails came out in the 1890s, all nails since ancient times had been of this make. They were flat-sided because a blacksmith had to hand-hammer them into a point, which meant the sides tended to come out flat. Such nails basically fell out of use around the year 1900 since the modern style wire-cut nails were easier to make and were cheaper to buy.
Here's an interesting suggestion for why coffin nails were once so common... they were apparently used for more than just nailing down the lid. They were, in fact, decorations.
This coffin, from a museum in York, is a replica of a 19th century coffin that was found in a church. It shows how the nails were used as decorations, over a cloth cover. Before French Polishing was invented, coffins oft times were covered in fabric for decoration.