I doubt that you will find a fake precious wentletrap of recent manufacture.
If any exist at all, they would have to be classifiable as antiques by now.
If any such counterfeits were created, they were made to capitalize on the
fact that real specimens were extremely rare and valuable - the same reason
fake Cypraea leucodon are created today.  But I can't imagine anyone going to
the trouble and expense of producing counterfeits of a species, when a large,
perfect REAL specimen can be purchased for under ten dollars.
One of my most embarrassing moments, some years back, involved a very large
("possible world record") Mitra hilli from the Philippines.  I got it from a
reputable supplier, who had sent me many very nice shells previously.  I sold
the shell to someone (fortunately a long-time friend who knew me to be
honest), who returned it with a note saying "check the spire under your
microscope".  I figured I'd see some small borings or something that I had
missed, but I wasn't ready for what I saw.  Under the scope, it was apparent
that several whorls of the original spire had been broken off.  The spire had
been sawed off flat below the break, and a spire from another shell had been
glued on.  The gap had been filled with some compound, then the whole thing
had been carefully smoothed, polished, and colored to match the lower whorls.
Even after seeing this under the microscope, and knowing exactly where the
repair was, I still could not tell with the naked eye that anything was amiss.
To add insult to injury, I don't think the replacement spire was even from a
miter - it looked like it might have been from a Turris or a Tibia or some
other elongate form.
Paul M.
Rhode Island