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.
This "forgery" is not exactly what
I would call a "forgery". It appears to be cut out
of one of the Spanish Centenary issues of 1950, or possibly a
photocopy of the stamp in black (None of the issues in the Centenary
set were actually printed in black!) and then cut out. This one
was being sold on ebay as a genuine unused stamp.
None of the background lines extend into the white
gutter, nor does it have the black "knob", as described
for a genuine stamp. The line that forms the throat, running
from the chin to the bottom of the base, is a continuous, inward
curve. On a genuine stamp, that same line would curve sightly
outward, then curve away sharply to the left. (comparative detail)