![Image](http://gimpchat.com/files/4643_map.PNG)
I found there was an inner discoloration and an outer discoloration.
I duplicated the layer and only worked on the top layer.
G'MIC > details > freaky details (because you lose print clarity with next operations)
Free-selected with fuzzy select the inner portion of stain. I used colors > curves to lighten it and then did the same with the outer stain. May have to do the text details again with freaky details.
Then adjust opacity and lower layer to original.