your point Mat, regarding the type of stem and knop construction, is worth mentioning. I've just had a look at Tim Mill's excellent book on Rummers - which is worth buying if you do collect this type of drinking glass - and I can't see an example showing a cushion/ball knop - plus collar - to quite match your stem.
Apparently strong knops were an Irish feature, but the rest of your glass doesn't show any of the known Irish characteristics.
However, the ovoid bowl shape is o.k., and together with the other features you mention would indicate this is probably right - certainly this type of knop can be found - on rummers - in the first 20 years of the C19, and it's possible your glass may be a little earlier than you are suggesting, assuming it is genuinely old.
Assessing clear glass on the screen is not easy, and age is determined often by looking at the combined features. The height is heading towards the higher end of drinking rummers.
Looking for deeply recessed dirt in pontil scars can help.
The potential pitfall with rummers is that they were made in the C20 between the wars (and in fact have been made in the last 25 years by Royal Brierley for example) and with snapped pontil scars - but the colour of the glass, seeds, tool marks and evident wear etc. should help to discount those. Let's see what others think.
