Hoping to confirm I've read these marks correctly and see if anyone has more insight as to the maker which Jackson and others list as unattributed. I read this as:
The mark also appears as Grimwade 3508, noted on a sugar caster of 1714, name still untraced.It is probably an unregistered mark of a known silversmith whose name begins Co. The outline of the cartouche has some similarity to the mark of George Cox but I know him as a specialist spoon maker and am not certain whether he was still working at this date. A long winded way of saying I cannot really help, but it is a nice piece.