Variations in the finish of the base are fairly random, Suzy, it simply depended on how busy they were and whether or not somebody had the time to do a more time-consuming finish.
I believe the oblong ones are far more unusual than the other tall ones, they also seem to be highly individual in how the neck is constructed - I have one with a straightish neck, and another with an extra sort of rounded lump around the rim.
There is a 10.75" tall amethyst bottle illustrated on p.41 of the book. I have a very tall one, but, right now, there is no way I'm going up a ladder to retrieve it to measure its height! I broke a retired Denby mug yesterday, I'm not feeling too good and am very clumsy and dizzy, and though I'm not superstitious, I ain't taking ANY risks with it. It is absolutely glorious, with masses of iridescence.
This is only the third amethyst one I've seen, counting mine and the one in the book.