Thanks for providing a photo of the back. That type of iron-back is strictly 20th-Century. It was developed as a very inexpensive way to mass-produce buttons entirely by machine. This back didn't have what we'd call a loop (or shank) ...just two thread-holes on opposite sides of the small dome at the center of the "stamped" sheet-iron back. This eliminates the need to make a wire-loop and attach it to the back ...thus saving time, labor, and material.
Note: This 20th-Century stamped sheet-iron back looks somewhat similar to a Colonial Era "turret-shank" back. But turret-shank backs were made out of CAST brass or pewter.
Regards,
Pete [P.C. George]