Hello Eduardo,
A Marginellidae specialist I am not, but I do have a good number of specimens
of both M. ventricosa and M. tricincta.  They are superficially similar, since
they are both about an inch in length, and uniform gray in color.  M.
ventricosa is more variable in color, ranging from dark gray to almost white.
M. tricincta seems to be more consistent in color, a dark gray, comparable to
the darkest M. ventricosa.  M. ventricosa is wider for its length, especially
at the posterior end.  M. ventricosa has four heavy columellar plaits (ridges
or folds in the columella, the central axis of the shell).  M. tricincta, as
you observed, usually has a weaker 5th plait, posterior to the other four.  In
M. ventricosa, the apex (tip of the spire) is visible, and can be felt with
your finger as a sharp point.  In M. tricincta, the apex is glazed over, and
covered by the posterior end of the lip.  You can't see or feel it.  From the
data I have received with various lots of these two species, M. ventricosa
seems to be a shallow water species.  The usual locality is India or Sri
Lanka.  M. tricincta is apparently a deeper water species, and mine have all
come from Taiwan.  I don't know the actual geographic range for either species
however.
Regards,
Paul M.