To your question Ross,
Perhaps Vice Craysacc dealer of New Caledonia may have the correct answers,
as he specialises and collects  melanistic and rostrate cypraea. I notice
no mention is made of the fact that just as many melanistic and rostrate
cyp and to a lessor degree conus are found on and off the central
Queensland coast on the same latitude as New Caledonia ( Tropic of
Capricorn )  Gives plenty food for thought.
                Cheers Dawn