There are several methods of registering/numbering shells that I have used or
observed that have been very satisfactory particularly if they are specimens that
would be spoilt by surface numbering. In my own collection each lot is either
placed in a suitably sized clear plastic bag with data inside the bag which is
then placed again in a small, open cardboard box. The other method used here by
several collectors is to insert a small label with full data inside the shell
which can be removed with tweezers when required. While this is difficult with
small shells, suitable small plastic containers with sealed lids are available.

I like Charles idea for coded catalogue numbering.

Kev