Hi Brooke,
Murex (Chicoreus) torrefactus is definitely distinct from Murex (Chicoreus)
brunneus, and once you get to know them you can separate them at a glance,
but it's one of those cases where it's difficult to give a really solid list
of obvious differentiating characteristics in print.  I'll try.  First of
all, M. torrefactus (Sowerby, 1841) is generally considered to be a synonym
of Murex (Chicoreus) microphyllus (Lamarck, 1816), so I'll refer to it by
that name.  M. microphyllus is generally a larger shell, commonly reaching
100 mm or more in length.  M. brunneus is usually about 50-60 mm, and 75 mm
is a big specimen.  M. microphyllus is more elongate, with a considerably
taller spire.  The color of M. microphyllus is usually a light to medium
brown, with fine darker spiral bands.  M. brunneus is usually almost black,
often with white showing along the trailing edges of the varices (the side of
the varix facing the lip).  In M. microphyllus, on the inside of the
posterior lip, between the largest lip spine and the body whorl, there are 3
or 4 small but distinct riblets, which are lacking in M. brunneus.  In M.
microphyllus, the longest body whorl spines are often on the anterior end of
the body whorl, while in M. brunneus the largest spine is always on the
shoulder, that is at the posterior end of the body whorl.  In M.
microphyllus, the three spines on the dorsal surface of the siphon canal are
distinctly offset from the spines of the body whorl (they do not form a
continuous line - the three siphon spines form a line of their own, which is
not in line with the body whorl spines).  In M. brunneus, all the major
spines are in line, from the shoulder to the end of the siphon.  In M.
microphyllus, thed; while in M. brunneus the canal is considerably wider (you
can fit a dime, edgewise, into the canal of an average-size specimen) and
nearly straight.  Hope that helps.  If you can't see any of these
differences, I suspect one of your lots is misnamed.
On your other question - in Murex shells, the juvenile shells usually have no
distinct characteristics that differentiate them from adult shells, except
size.  So, a shell that looks like a tiny Murex brunneus is probably just
that.  I have one that is only 21 mm, and it looks just like the full grown
ones.
Regards,
Paul Monfils
Rhode Island, USA