Now this is just a shot in the dark, so please don't shout me down!
I have been looking at some of Siddy Langley's work, and she does a colour combination called 'Pheasant's Feather', that looks extremely similar to this 'Shell' colour way. She also makes a lot of long necked bottles, and just supposing she was making one and the glass slumped as she was drawing the neck up, might she have decided to play with the glass to see what happens, and ended up with this 'Shell thing'? Much as aa and Furhman Glass have intimated.
Just a thought anyway, and as she has a well illustrated web site, so, could it be worth sending her an email with a picture of your 'shell thing' just to see if she remembers making it? If it was just a piece she was playing and experimenting with, she may have decided not to sign it.
Here is the website address and if you scroll down you will see the 'Pheasant's Feather' pieces:
http://www.siddy.com/black-website/Images.htmlAlso on eBay there is one of her pieces in this colour way with a picture of the base which has remarkable similarities... do look and see:
http://www.ebay.co.uk/itm/Siddy-Langley-superb-Pheasant-Feathers-Flask-/380813555448