I've been doing a bit of searching for something else and revisited this old, long and now locked thread.
There is one picture still on there that I think is quite possibly a Stevens and Williams shamrock piece,
Source - Charles Hajdamach's 20th Century British Glass page 36.
The patterns are shown as pattern numbers 30630, 30631 and 30632 and date to December 1902
On the pieces in the book the shamrock points down and is placed on the side wall of the vases with its 'stem' pointing upwards.
On the piece in the Glass files on here (see link), the shamrock points up because it is placed over the top projecting two opposing raised 'corners' of a 4 cornered bowl. I think it is the same pattern as the book but has been placed in a different direction to suit the shape of the bowl.
Another member has commented on the locked thread saying that bowl looks like a Stevens and Williams 'grotesque' piece so the S&W connection is also there with the shape.
http://www.glassmessages.com/index.php/topic,1901.msg49483.html#msg49483the link to the piece in question is here
http://glassgallery.yobunny.org.uk/displayimage.php?pos=-2180m