the P. of W. feathers are quite unmistakable, and I can't seem them here - is it worth looking through Burke's Peerage, or similar, or perhaps you already have. Might it be that only those glasses made for, or owned by the P. of W., would show the three feathers - there used to be an image of them on a Sterling 2p piece. For those who have Tim Mills book on rummers - there's a very good pic of those feathers on page 19 - figure 16.
Not sure what to think re the date - you could be right, or maybe earlier - these hollow stemmed glasses seem to cause more trouble in dating that any other, but can say that the ground/polished pontil depression doesn't help to tie down the date.
Script/font doesn't help either - just looks typical of middle third C19, or earlier.
Is there any chance that symbol might have a masonic connection?? can't really make out what it's supposed to represent - but it may well hold the key.
sorry this is no help - but nice glass though.
