color omitted ?
Monaco # 26, black /yellow. The one on the right has the yellow omitted, could it be that the color just faded away or was the yellow omitted ?
Yellow is highly susceptible to UV exposure. If this has set out in the sun for a long time, could easily have faded out. The black certainly isn't as bold as the one at left either, so I would say very strong probability. In fact if you look closely in the diamond shapes in between the two figures on the stamp, it has a yellow tinge to it, so this suggest faded. Same above the 1f.