Eh, it is always frustrating to be shown only a mark and given minimal information. Is there a reason you can't post a picture of the cup? The small bit that can be seen seems to show plating wear revealing copper, is that so? Might the piece be French in style?
From what I can make out, the mark looks like it may be '20ME', a mark that would have been used on French fused/rolled plate after 1797 - the excerpt below is from Okie's 'Old Silver and Old Sheffield Plate' (1928}...
~Cheryl
