Mystery solved (maybe). Cases are OAK and leather. Leather is hygroscopic, and oak has tannins that will turn ferrous metals black. That's why you seldom see steel or iron hardware on vintage oak furniture. The usual brass hardware was not just for looks, it was to keep the tannins from attacking steel hardware. Topic is covered extensively in vintage cabinetry and contemporary refinishing books. The moisture attracted by the leather in a relatively humid climate combines with the tannins and creates the black color on the guns.
Where are the chemists?
