Square Nails, my favorite subject. I love them because then I know I got an old one. Yes, nails have been around for over 1000 years. The earliest settlemensts of the new world show evidents of construction with nails. the shape of the nail in our furniture is a good indicator of age . The earliest nails found in antique furniture were made by making a tapered shanks form raw stock. To make the rose head the blacksmith would make 4 strikes with the hammer. Early in the 19th century cut nails were made with a square head and a spatula tip. Not until later in the 1800’s did we invent the wire nail that look similar to the modern round nail.
And yes you can put square nails in a new piece. Therefore, it is important to look for appropriate oxidation and proper staining of wood around the nail to assure of good real age.