Hi, the darker ones were made with a ruby core as opposed to a flint core maybe earlier ones? not sure but maybe another reason why the larger bubble was omitted 'cause you can't see it anyway.
Did you buy one of the two.
Don't really keep up with Whitefriars sales now, though it is a good time to buy but dealing wise sadly died a death over the last 4 years. I can remember 5 years ago buying every piece of W/f that came up knowing it would sell, at one time I sold nothing else, now unless it's something special I don't even pick it up though the textured range and the millefiori weights still do well so too some of the rarer colours.