Some very early RC papers weren't totally stable and there can be image deterioration which can't be reversed. If it's definitely early 80's it should be on the newer versions. The yellow stain may not disappear in fixer/hypo, it usually requires a more concentrated solution than usual which is also likely to attack the highlights of the image.
There are special techniques to remove these stains, but you need plenty of experience before attempting them. As others have said refixing the image is a good idea. It may not remove the stain but could prevent the image deteriorating further, on the other hand you might be lucky.
Ian