I'm not sure anything was applied to the case-hardened area of (vintage) American made doubles at their manufacture. I've never read or heard anything addressing that. However, it is nothing short of amazing how well the colors hold up in the most protected areas. This is the forearm of my Philadelphia made 20 ga. Fox Sterlingworth. These colors are absolutely 100% original. I have refinished the wood. I don't have a picture of the action for a comparison but suffice it to say that there is less than 10% case color remaining on it. Each time I remove the forend I try to imagine what the action looked like when it left Philly with such vibrant colors on it.

[Linked Image from jpgbox.com]

May God bless America and those who defend her.