Hi - not a Prussian in my opinion, but far more likely to be an Indian Club - which is a less common shape - the Prussian it seems was a very common shape which bridged the C18 and C19, but the difference is very obvious when the two are placed together. Prussians are fatter in the body, and the outline shows more of a shoulder, whereas the Club is of a slimmer shape has a softer and more gradual line.
This piece shows examples of ornamentation of late Georgian decanter shape and styles, but these are not the more common ones you'd expect to see in the first thirty years of the C19.
Starting from the top...............
Typical stopper shapes would have been mushroom, lozenge, target, bull's eye etc. This heavily cut stopper is not unknown, but not perhaps the norm for the period.
Does the wear inside the neck match that of the stopper stub. I doubt that there would be matching Nos.
Neck rings would have been applied separately from c. 1790 to something like the mid 1830's.
Have a look to see if there is the appearance of a crack between the rings and the body, which should indicate that they've been applied, rather than being part of a moulded design. Again, the cutting on these rings is not unusual for the period, but not that common.
Those decanters without star cut bases - for example the Georgian spirit squares - show a base with a large but shallow depression where the pontil scar was removed. A completely flat base like this one might indicate a Continental origin.
Are the horizontal cut mitres very symmetrical, or slightly irregular in their shape?
Unless you're very lucky, it's likely that the points of the relief diamonds will be damaged - if they're in exceptional condition I'd be a little worried.
At the base of this one I think I'm seeing mitre flutes rather than the more common blazes (either slanting or vertical). Blazes were less deep I believe.
At the end of the day, colour and wear may well tell you more than text book period features, and it's back to that old instinctive gut feeling as much as anything else.
Plenty of very good copies of Georgian glass made post 1880 from the U.K., and some very good pieces from Bohemia early in the C20.
As always with these things almost impossible to be conclusive based solely on a screen image.
