It almost looks like a burn. Either from heat (?) or some type of solvent. Like the Browning salt guns, something has reacted between the metal and the wood. Could it be from someone using used motor oil on it, yet it never soaked the wood? I know a lot of the old timers used it on guns. Have you ever removed the trigger guard screw to see what the wood looks like down in the hole? If you have, is the wood rotted out deep down in the wood? Or just on the surface? Or could it be some type of fungus that didn't grow until after it left the plant? Surely it didn't leave the factory like that.
