Hi Peter, thanks once again for your authoratative response, and also for the Dutch name of this item and links. Taking each of your points in turn:
1) Rotterdam: Indeed, it's obvious now you tell me!
2) Rotterdam year script: Tardy says that for the years 1783-1807, the letters run A-Z, which for 1805 would suggest an X to me, being English, but now I look at the Dutch alphabet on the web I see 'ij' is another way of spelling 'x' ! Is there a chart on-line showing these pre-1808 marks?
There's also a mark on the piece above the city mark, a 'y' with a full stop above, which I took to be the date letter - what is this?
3) Crowned O: I thought that might be the taxfree mark of 1807, but the crown seemed more 'pointy' than shown in Tardy
4) Crowned V: Even I was able to identify that!
5) Maker's mark: I've attached a picture below with the mark in different positions since it isn't clear to me which way round it goes. The thing measures just 2mm in diameter, so is incredibly difficult to see! It's seems to be in script form. In one orientation it looks like 'SC' and then if you invert it looks like 'JS' or 'YS'. There also appears to be a 'c', measured in nanometres.
Given the various non-overlapping dates: 1805, the taxfree mark of 1807-1810 and the crowned V mark for 1814-1893, what do you think is the story here?
All the best,