Have you looked into the possibility of yer shell being Perotrochus hirasei?  All of the pics I've been able to find of it in my books and internet all seem to have a shortish slit, like you were saying.  Of course, it'll vary by the individual, but in relation to others in the genus it is shorter.  I admittedly don't know much about this family, although I would LOVE to collect them.  But P. hirasei is my first guess based on yer picture.

Are there any ID keys or useful books for this family (Pleurotomariidae)?  I know its really small, and most of the sp are considerd rare.  In all my shell books they're just kinda tagged onto the front, if they're included at all.  Has much work been done on them?

~Stephen~