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Comparing the date letter cartouche and the lion passant, I'm going to sticking my neck out and saying I think its the letter f for 1781. I believe this was the final year for bottom struck marks too.
1781 would support the maker as being George Smith III, listed as a spoonmaker, whose mark was first entered 1st Feb 1774 (Grimwade 906, p74). The picture of his mark in Grimwade is a good match and he has added the note that "Various similar marks 1775 - 82".
I also checked the photo Phil has on his site and the marks are similar but both the G & S have loops on the bottom in his example which are not on yours but this may be one of the variations noted by Grimwade.