Hi Tom,
Well, I can't confirm or deny that story from any authoritative source; but
from a purely logical viewpoint, I very much doubt it.  How could this be
known?  You couldn't tell from looking at a large bulla form cowrie whether
it was a "permanent bulla" or one enroute to the adult form.  So, even if it
were true that some large bulla forms remain so, I can't imagine how this
would have been discovered??
I did have one specimen though, that remained a bulla - sort of.  I had a
large Cypraea arabica in which the thin lip of the bulla never turned under
or developed teeth.  But the shell, including the lip, did increase in
thickness, and develop adult pattern and gloss.  It had teeth on the
columellar side, but not on the lip side of the aperture.  It kind of looked
like a big Oliva bulbosa with a Cypraea arabica pattern.  This was just a
random malformation though, I am quite sure, not standard practice.
Paul M.