Hi Stuart - I wasn't rubbishing your rummer, but suggesting it wasn't from the 'golden era' - it may be made well, but I don't think it's earlier than late Victorian - early C20.
If you are saying it's without any form of pontil scar or ground/polished depression then I'm of the opinion that it is not going to be earlier than I'm suggesting.
I wasn't suggesting that it's deco in style - pan-topped designs have been around for a long time - just that it may have been made during the period when deco flourished i.e. 1920's - 30's....but made to look late Georgian/Regency (1800 - 1830).
Earlier in my collecting days I bought b.b. and b. gr. rummers (with snapped pontils) only to discover they weren't genuinely old ...... in fact I also bought a couple of clear ovoid examples of equally dubious date.
It's a well documented fact that at various times from the 1890's to the mid 1930's there was a lot of copying of classic Georgian and Regency glass - that by now will show substantial wear.
I'd like someone to prove me wrong, but I think unlikely in view of some of the features we've discussed.
This may be British, but might equally be from the Continent - difficult to be more accurate.
Did you buy this as an early C19 piece?
I'm hoping that someone else will comment.