Its cutting edge conically-shaped, a Bit designed to cut conical or circular beveled recesses for countersinking Screws.
These images show a selection of countersink bits. (I have an image of a bored countersink hole to be uploaded later.)
The larger image on the right, below -- actually a jpg for a page from a book in the Making of America database, is the result of a quick-and-dirty search for "countersink": Shelley, C. P. B. (Charles Percy Bysshe), 1827-1890, Workshop appliances including descriptions of the gauging and measuring instruments, the hand cutting-tools, lathes, drilling, planning, and other machine-tools used by engineers New York: Appleton, 1873, page 64.