Well, now I’ve completed my research, I can confirm that this cup was indeed made in Waterford. Not American at all! It was made by either Anastasia Fleming, or her son, Ignatius Fleming around 1800. The prefix letter referred to by Dognose is confirmed as a cut-off ‘A’.
In fact The Bishop’s Palace at Waterford in its ‘Waterford Museum of Treasures’ has an extremely similar cup in its collection. You can see this at the following site, though you’ll need to be able to run Java properly to see the video of the cup:http://www.askaboutireland.ie/reading-r ... ilverware/
The museum cup has a monogram which I read as ‘SK Junr’ and mine has ‘SK’ as part of its monogram. A certain Samuel King was Mayor in Waterford in 1786, 1800 and 1820 and he lived at Mt Pleasance, so it’s very likely there’s a connection here.
Thanks to all those who have helped me in my research.