Hi , I dont think your glass is as mysterious as you think ,I would say it is a very large rummer,possibly for ceremonial use ,ie filled and passed around at a gathering , a civic ceremony, a marriage or such like,or merely a very large glass for someone who likes more than there fair share ,
For those who will be visiting the Cambridge Glass fair this weekend
www.cambridgeglassfair.com you will be able to see in the Foyer exhibition a large collection of goblets of all sizes,some with a capacity of more than 2 litres,from the 17th and 18th centuries.from my collection.
Peter.