I've been trying to come up with a list
of characteristics to help distinguish a genuine 6c from a forgery.
But there hasn't been, at least to my knowledge, a problem with
forgeries for this issue. The modern day photographic forgeries
on thin paper are about the only ones that might fool someone,
and only because they are replicas of genuine stamps.
The one detail that seems to be lacking in all the forgeries
I've seen is this:
The background mesh is comprised of horizontal and
vertical crossed lines. There is a thin white gutter between
the central area of corssed lines and the outer frame. Some of
these crossed lines that form the background mesh extend beyond
the confines of the central area into the thin white gutter (detail).
There are some extended lines that are consistant on each and
every position in the plate, such as the 2 lines just under the
numeral 6 in the top inscription (detail) and others are unique to particular
positions. Some are not as distinct as others, but every 6c will
have some. The modern forgeries also show some of these lines,
but they are not as distinct.
There is also a small "knob" of black color
in the upper left corner of the central design area, that protrudes
into the thin white gutter (detail). It seems to be missing on most forgeries,
with the exception of the photographically produced replicas
of modern times.
Like forgery #2, I believe this could also have
been cut out from a book, album, or catalog.
It is missing the background lines that extend into
the white gutter area. It does not have the small black "knob"
of black, in the upper left corner of the background mesh area,
as described in the genuine stamp. Also, notice that the corner
ornaments are not quite the same as wthe on the genuine stamp.
The face lacks detail and depth.