Hello again,
All I can tell there is a key in the central mark, but nothing even close to be found in Tardy, nor Rosenberg.
Being benevolent (this is what we are here, after all) let me tell something about the craftsmanship of the caster; to be honest — not well made. It has nothing to do with the period, on contrary, silversmiths were more skilled in the past, and actually the quality of silver steadily decreased after early 19th century (with exceptions, of course).
The baluster shape is not graceful, the cover is poorly pierced and chasing and embossing, as well as the finial are not well executed.
There is a beaker posted here; crudely made, too (different marks).
If both items come from the same source I would be very suspicious, but I may be wrong about the source.
In short, my humble advice is ``buy what you really like``.
Examples given here to compare: covers of late 18th and early 19th century continental sugar casters.
Typical embossed and chased ``Wiener Rosensilber`` décor (Vienna rose silver).
Kindest regards
