I had some work done by this company over 25 years ago. Don't know what they do now, or if it's the same owners.
The Color Case Company
New Springfield, Ohio
Phone: 330-542-2062
Case Color Co owner, Don Menk, retired a few years back and closed the company down. AFAIK know one bought him out or continued his business.
Great guy and it was a great place to get the work done.
Add: RGS Restorations (Rons Gun Shop) to the list of those doing nice color case work
http://www.ronsgunshop.com/All types of finishing available.
Heinzelmann's in NJ does cyanide case hardening. The color pattern is noticably different than bone & charcoal case hardening.
Heinzelmanns has been around since WW1. (They are the company that A.H.Fox of Phil,PA sent their shotguns to for case hardening when AHF switched to cyanide hardening.)
Early AH Fox was bone/charcoal hardened.
Remington production Parker shotguns were cyanide hardened and show the difference in pattern from the earlier Meriden,Ct produced guns.
Remington did their own cyanide hardening I believe as they used it on small parts as well like triggers and cartridge guides.
All that matters in the shotgun restoration world, but probably not so much here.
But it's a point worth noting I guess if small details like that are of concern and they generally seem to be.
I don't know if potassium cyanide (the common chemical used in blacksmithing cyanide hardening) was available, where, and during what times the M/L's discussed here were made.