You might try Efferdent or rubbing the stain with a cut lemon and putting it out in the sun. The first is surprisingly effective in removing some stains. After all it was designed to remove stains from false teeth. I have tried it on old discolored and stained lace and it worked well.
I used lemon juice and salt and putting it out in the sun method on a lace and satin wedding dress from the 1940s/1950s. It did not get the stain out completely but only left a faint trace.