I just got a 1770 London sterling tankard. The dealer believed the mark to be Jacob Marsh. The auction from where they acquired it identified it as either Marsh or John Moor (Grimwade 3658).
I personally think it's John Muns (Grimwade 1518) based on a previous discussion on this board: viewtopic.php?t=6238
The date of 1770 for this tankard seems to fit well with Muns.