The two J&S Hawken rifles I examined closely had patch boxes that are now appearing as browned. I rather think that practically, it would make most sense to case harden parts that face wear in use, or in some cases, to better resist rust. It seems that hardened surfaces pit less. Also, we know that many J&S Hawken matchboxes were engraved either simply or fancifully with animal figures. Case hardened boxes could not be engraved after hardening. So it would all seem a lot of work to fit it, engrave it, case harden it, and hope it doesn’t warp. If it does, what’s the cure? Tempering beyond spring temper, so it could be re-shaped. Then start over.
I might be over-thinking this. Wouldn’t be the first time!