The story I read once went something like this. Somebody at the Dunhill factory was moving a box of loose sandblast pipes and they were rattling around and someone said something to the effect of " It sounds like sea shells rattling" and the name shell stuck and it refers to their sandblast pipes. I read that somewhere a long time ago, probably in the Pipe Smokers Ephemeris; the exact details may be somewhat different but that's what stuck in my mind.