Staining due to rust is the most likely explanation . I used to make a black wood stain dissolving iron filings in vinegar.
Very difficult to get rid of entirely but bleaching with oxalic acid is a good start . Try looking at the wood work /antique restoration suppliers ,there are a number of bleaching products available . The result may be that the wood ends up "white" but can be easily stained back to match ,but go gently with the stain on the treated area as it will almost certainly darken quicker that the rest of the wood .
