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.
Lets see what the others have to say.