Case Coloring
Winchester, Marlin, Colt, those are just a few names that used this finish. In reality it was the basic receiver finish for most manufactures of handguns, shotguns, and rifles. Commonly used till about the turn of the last century.
Case coloring provides a durable hard wearing "skin" to the malleable metal underneath.
This finish is created using heat and mixtures of charcoals with a quenching cycle at the end.