I have a Scott #80 stamp - King Luis 10R overprinted PROVISORIO that is a very dark color. Most of the enamel coated stamps I've seen from this series have a shiny white or a clear sheen. But, this one has a very significant discoloration. Has anyone else seen anything similar in their Portugal stamps? The picture of the back and the close ups of 2 corners show it to be the coating that is discolored.
I thought it was just dirty too. Tried a water bath with some soap and also tried a soft white eraser when it was dry. The pictures I took were after those efforts. In the zoomed photos the perf tips look like the enamel coating is actually discolored while the paper is still white.
Oh well, thanks for checking.
It may be that if the discoloration does occur collectors have treated them as heavily soiled stamps and probably discarded them.