Just a few thoughts Rob to start this off, and hopefully this will encourage the knowledgable people to come in.
There is a 'shaft and globe' example shown in Douglas Ash's 'English Drinking Glasses & Decanters' (1680 - 1830) - in flint (and apparently in the British Musueum - although now possibly in the V.& A), which is dated A shorter neck than yours, and with what may be the more common(!) application of 'nipt diamond waies' - although these may well have varied in style. The neck ring is presumably a left over from the bottles that the decanter superceded, as anchor for the string, and perhaps your eleborate brass stopper was a later addition. The example in Ash is shown without a stopper/cork, and rests on a rim foot, although looking at your decanter, what I'm seeing suggests a standard non-footed base. Coincidentally, it seems that the word decanter came into use in about the 1690's
According to this author, all flint decanters at this period had a 'kick' in the base (in common with serving bottles) - to provide a stable base plus somewhere to hide the pontil mark - although I appreciate you are saying that as your has a ground top then there won't be a pontil.
In Bickerton - under coloured glasses - there is a single 'purple' wine glass dated (probably by the V.& A.) to 1690 - and against which Bickerton has added an exclamation mark in brackets. In the Georgian (British) period - 1740 plus, purple/amethyst was a not uncommon colour (now very collectible) - but how far back prior to that period it goes I don't know. Am sure that Peter (oldglassman) might like to answer that.
You can of course contact the V.& A. museum in London, who are usually very helpful - even if you do sometimes have to wait for a week or two for the answer.