This chisel has a hexagonal socket, a feature which seems common to 17th century English chisels. The body and socket of this chisel were made out of a single piece of wrought iron. First one end of the bar is forged into a flat triangle shape and welded into a cone. The cone is then shaped over a mandrel made especially for the job.
All chisels made during the 17th century had a wrought iron body with a piece of steel welded on the bottom. Here you can see where the iron ends and the steel begins.