Hi David,
' But what is it in the picture that helps you date it? ,'
mmmmmmmmm this could lead to a ramble but i will try not to,Georgian drinking glasses even those not made of the best metals but in the form of those which are considered fine glasses have always been considered to have a form and proportion that is very pleasing to the eye,there is I know a lot of very academic works on the subject of why art forms in architecture and other constructed works of art ,glass included,seem to have a natural balance and pleasing form to the human eye ,creating an appreciation for the objects, for me though being a mere mortal its a case of having been around probably thousands of 17thc and 18thc glasses for a goodly number of years, those forms become fixed in the brain and when something which 'just don't look right' appears ,you look closer , but it also helps that most of the repro type glasses have also been seen in the hand ,so those things combined help spot the fakes.repros etc
For the glass on ebay ,to my eye the stem is to short for the size of the bowl,out of classic proportion for a glass of this type,the foot looks a little flat compared to a period conical foot and the fold looks like it may be a little fat for my liking ,and even with the bad photo the metal looks far to clear and bright, which are all pointers that have been noted on repros of this type that I have held ,
Anyway ramble over hope it helps a little,
Cheers ,
Peter.