my thoughts for what they're worth are.............
Is the pontil scar right for the early C19? - period pieces mostly show a scar that looks more irregular and possibly sharper than this one where they have been snapped off from the pontil rod. This one appears to lack the signs of a break, or snap, and reminds me a little of the Continental mark, where the scar is very neatly circular and not rough, and is achieved by using a hollow circular pontil rod rather than a solid one - but of course it may well not be Continental.
grape and vine engraving seems to be very unusual for a period rummer, which was a type of drinking glass used mostly for beer/ale/watered rum - I've looked through the books and the only example I can see is the one in Tim Mills book. So not unheard of, but very rare.
Ward Lloyd ('Investing In Georgian Glass') says of the rummer..... "hop and barley engraving is not unusual; this would seem to indicate that rummers were basically intended for beer drinking".
We're speaking here of drinking rummers of course, but it's possible that grape and vine might well have been a motif seen on the much larger serving rummers which were used for the preparation of punch and wine beverages.
However, it's mostly probably back to Neil's comments about colour and wear - there's something about a glass that is c. 200 years old that speaks of age, and probably more so with a rummer - perhaps they pick up more age related wear than other types of drinking glass.
Sorry, no more conclusive than when we started I don't think.