I've come across two instances of a certain Palestine Mandate stamp documented as being - among other things - on thick vertically ribbed white paper, but here on brownish paper of the same ribbed type.
On close inspection of the perforations it appears the stamps are brown on the surface and lighter beneath it (based on the exposed fibers on the teeth). I would be tempted to think that the stamps are merely "toned" but in one case where the stamp is accompanied by two other normal looking stamps on the same (well preserved) cover, this would be an odd occurence if the paper is merely toned and not actually a different type of paper than documented -?
The image shows a single brownish stamp on a postcard above a cover with another brownish stamp and two regular white stamps. Any ideas?