Thanks for the pic David, looks as if it may come from the same family as mine too has a flat very polished base...
and Max..sorry but for some reason I can't open the photo you have kindly posted, it doesn't even flicker when I click on it...probably my computer which has it's own childish personality and will not grow up! Needs nursing along a lot. Usually when it doesn't like an attachment , windows pop up all over the place telling me so, but in this case you little icon just stares back at me rather sulkily I have to say. Not to worry. If anything comes to light on this type of glass, I'll let you all know.