Hi Bob and Rosemary,
Your Lambis sounds like an immature specimen that has not yet developed the
spines on the lip. Is the lip rather thin and fragile?  That would be
another indication of immaturity.
Murex duplex commonly exceeds 150 mm.  In fact, an African Hexaplex that
size almost has to be H. duplex.  However, I wasn't aware that H. duplex
occurred in southeast Africa.  I thought it was restricted to northwest
Africa??  I have received large specimens from Mauritania and Senegal.
Regards,
        Paul M.