I recently got a 12/44 Inland from a guy. The stock was pretty dirty and "Black" around the handle, I did a cleaning on it and it came back very dark chocolate with no dings, I also found a std products mark just out of the slingwell, Did standard products use a diff wood? it is much darker then any of my other guns.
The wood being dark is from oil, grease and plain old dirt and sweat. Standard Products used walnut like the other makers. If you want to clean up the black then try something like a good furniture stripper like Formby's.
Here's before and after pictures using a product called Murphy's Oil Soap. It's a citrus based cleaner that won't damage your stock and leaves cartouches and stamps intact. Notice that it did not remove the traces of paint stenciled on the stock but removed all the grime. Just allow it to dry and re-oil with flax seed oil (same as raw linseed oil).