Presumably it's the noticeable 'edge' to the white lettering at the top of the stamp?
Glenn you are right on there.
Michel states that for 802II
the upper frame is brighter
whereas for 802III
the frame is evenly toned.
On the other hand Michel states that 802II
(brown-lilac) and 802III
The former is obviously darker than the latter so something is wrong here.
Actually the Austria Netto Katalog lists three
different colour shades for the photogravure version.