The green crud is called verdigris, it's the same green crud that forms around brass snaps and rivets on leather gear reacting to the tannin in the leather.

Stock monograms are nickel plated brass, the nickel is worn and the brass is reacting with the tannin in the wood. I use Balistol in a needle oiler to neutralize it, then brush it off with a tooth brush.

After cleaning, I use the Balistol as a barrier film to prevent the verdigris from growing back.

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Conrad
SWCA #1830 SWHF #222