Craig
had similar probs on some s45/2's some time ago - followed the route outlined by Chris - I used oxalic acid solution(caution very poisonous usual disclaimer, however just use pox doctors gloves - latex!) to bleach the stain it worked very well but did not remove the ring completely however,then used boiled linseed oil / wax/ very fine wire wool to renovate - it all seemed to work well so much so when sold on the buyer thought the boxes were in superb condition (fot their age!)
Bon chance - hope it goes well!
salut
t
Just remembered another less strong bleach - salt crystals on the stain then add citric acid(lemon juice) let it work see if in fact it does work - I've used both the above to take stains out of decks on classic sailing dinghies - another of my passions!1
I always try to operate using/following the KISS principle -- Keep it simple stupid!