It looks like an "e" to me, therefore 1780. If it was 1789 it would not be bottom marked and would also have the Duty mark. George Smith entered this mark in 1774 and by 1786 had entered another mark as he was in partnership with William Fearn.
Definitely George Smith III. If you have a look at http://www.silversugartongs.com/london/ ... 0marks.htm there is a page of his marks. This particular mark is from around 1780 & my view is that the date letter you can see is a slightly squashed "e" for 1780/81.